There are three common answers to this question today.
Real Human Death. John says the soldiers pierced Jesus’ side because when they came to hasten his death they unexpectedly found him already dead. The piercing is thus the soilders way of answering the question, “Has Jesus truly died?” John may have wanted to definitevly answer this question because some in his circle claimed that Jesus only appeared to have a body (1 John 4:2, 2 John 7) and/or to have died. And indeed several recent studies have shown the flow could have been the result of purely natural causes. John cites two scriptural fulfillments of this event (19:36-37). The first of which says "not a bone of his will be broken." This is a reference to the Passover lamb (Exodus 12:46; Num. 9:12; Psalm 34:20). However, the referant is the unfulfilled intention to break Jesus’ legs. The actual piercing and flow is not specifically related to this typlogy.
Release of the Spirit. Water is an essential core symbol of the Gospel of John. We find it connected with baptism (1:26, 31, 33, 3:23), purification (2:6), tradition (4, cultic healing (5:7) and cleansing (13:5). John contrasts these earthly waters of purification with Christ’s offer of “living water” which he explicitly connects with the Holy Spirit (7:37-39). The flow in John 19:34 is a special fulfillment of this later verse, where Jesus' claims, "rivers of living water would flow from within." There, following his claim to be the true Exodus manna (John 6, Exodus 16), Jesus makes allusion to the water from the rock (Exodus 17:1-7). Here in 19:34 the flow points in like manner to the later day fountain for “sin and uncleanness” in Zachariah and Ezekiel flowing from Jerusalem and the temple. John’s second scriptural citation “they shall look on the one they have pierced” (Zach 12:10), found in the context of this escatological fountain, appears to support this conclusion.
Symbols of the Sacraments. To a lesser extent, interpreters continue to debate the merits of a sacramental symbolism in the flow of blood and water. Water has indeed been associated with baptism in the Gospel of John (John 1:33,35) and blood with consuming Jesus’ body (John 6:53-56). What confuses those who argue for a sacramental symbolism is the order in which the elements appear. They neither match the historical occurrence of baptism and death in the life of Jesus or correspond to the spiritual practice in life of a believer. Most scholars conclude that if John is making a reference to the sacraments it is secondary at best.
But there is another explanation which doesn't rule out these popular explanations and appears to tie much of John together. The early church saw in John 19:34, the piercing of Christ’s side and subsequent flow of blood and water, an allusion to Eve’s creation (Genesis 2:21-22). By the end of the second century we find Tertullian saying,
If Adam was a figure of Christ, the sleep of Adam was the death of Christ who was to fall asleep in death; that in the injury of His side might be figured the Church, the true mother of the living.
According to Alban Maguire,
This teaching had been foreshadowed before the time of Tertullian, and after his time we can find no doctrine more honored among the Fathers and Doctors of the Church.
How can the piercing of Jesus side and the flow of blood and water (John 19:34) have anything to do with the creation of Eve (Genesis 2:21-22)? The verbal and circumstantial parallels between the piercing of Christ side (John 19:34) and the creation of Eve (Genesis 2:21-22) boils down to these:
Death as sleep. The piercing and flow follow the death of Jesus. Genesis tells us that prior to taking Adam’s side to form the woman he caused Adam to go into a deep sleep (tardema). Deep sleep is commonly used of a night’s sleep as in Job 14:13; 33:15 and Proverbs 19:15. But here in Genesis 2:21-22 it is the same special work of God as when Abraham slept before the covenant with God (Genesis 15:12). Since Jesus rises again, His death is likewise comparable to sleep. Jesus makes the comparison between sleep and death explicit in John 11:11 before the resurrection of Lazarus.
Opened side. While “rib” is a good rendering of what God took from Adam in Genesis 2 it is unnecessarily restrictive. The Hebrew may refer to a rib but it more broadly means side. Besides it use in Genesis 2, it is used in the Old Testament for the sides of objects, buildings and hills. Side was also in the minds of the Greek LXX translators when they rendered the Hebrew as pleura. The substance God uses to form the woman is found in the man’s side. John 19:34 and Genesis 2 also share the unique fact that plerua is in both instances is singular when it normally occurs in the plural. It is the side (pleura singular) of Christ where John locates the piercing. Just as God takes from Adam’s side (pluera singular) so the solider pierces Christ’s.
The Substance. In Genesis God takes a part of Adam to fashion a helper just like him. In a similar fashion, what flows from Jesus side is a representations of his two natures. The blood stands for his flesh or humanity while the water His spirit and divinity. You can somewhat see this in the standard views on the flow of blood and water reviewed above. Blood refers to Jesus' real humanity while the water represents the Spirit.
These verbal and circumstantial parallels are by no means definitive. But strengthening the connection are the multiple themes in the gospel which point to it. The greatest support for the connection may not necessarily be the ‘textual’ similarities in 19:34 but rather how it corresponds so neatly with John’s message and themes.
New Creation. As a significant reference to creation this interpretation matches John's numerous allusions to Genesis' creation account. A new creation theme begins with John's opening allusion to Genesis 1:1 (“in the beginning”). John ascribes the creation of all things to the Word/Jesus (1:3) (1:14) and so declares that what came into being through Jesus was a new creation – a new beginning. As the author of creation, Jesus is the source of life and light (John 1;4, Gen. 1:3). And in the gospel we see Jesus offering life light to the man born blind in chapter 9 and life to dead man Lazarus in chapter 11. Jesus' Sabbath "work" is also tied to the creation narrative. When people object in John 5 to Jesus’ healing on the same day God rested from creation, Jesus responds, “My Father is working until now, and I am working.” (5:17). Jesus implies that neither the Father nor himself has ever stopped working. For Jesus, in John, there is yet to be a completion to the creation week. That is until Jesus declares “It is finished," echoing the completion of creation in John 19:30 (Compare John 19:28-31 to Genesis 2:1-3). John also points to Genesis by calling the place where Jesus was arrested, crucified, buried and raised a "garden" and by depicting the disciples reception of the spirit with similarities to God "breathing into" Adam the breath of life.
New Birth. This interpretation makes sense of John's theme of new birth. The creation of Eve is the first birth recorded in scripture. The term "born again" comes from the gospel of John, when Jesus tells Nicodimus "no one can see the kingdom of God unless they are born again" (John 3:3). But that's the only place where the theme of new birth appears. It forms the pivot of John's 18 verse interoduction, "Yet to all who did receive him, to those who believed in his name, he gave the right to become children of God— children born not of natural descent, nor of human decision or a husband’s will, but born of God." (John 1:12-13). And in John 16:21-22, Jesus compares his disciples expiernce of his crucifixion with a woman in labor. Indeed, it appears that John is depicting the flow of blood and water as a birth.
In this 2005 JETs article, J. Bergman Kline notes the following.
Elucidating more full the (re)creation symbolism of the blood and water flowing from Jesus side are certain OT idioms in which "what comes out from one's side refers to offspring. The following expressions refer to children or descendants: "that which comes out from your loins"; "the fruit of the belly/womb"; and "one who will come forth from your inward parts." In both the LXX and the the NT, koilia (which occurs in several of the aforementioned idioms) commonly refers to the womb.
The Phrase "the one who comes forth from the belly," can be equivalent to "the one who comes forth from the sides," as can be seen from the fact that each may refer to a son. This possible interchangeablity between kolia and pleura suggests a close relationship between Jesus' statement that "he who beleives in me...rivers of living water will flow from his belly (koilias)," and John's statement that "immediately blood and water came out" from the pierced side (pleuran) of Jesus (John 19:34).
Marriage. This interpretation makes sense of John's marriage theme. The creation of Eve is the first marriage in scripture and in John allusions to marriage appear every time Jesus engages a woman. At the wedding of Cana (2:1-14), John casts Jesus as the bridegroom and later echoes a betrothal type scene (Genesis 24) in Jesus conversation with the woman by the well. John again echoes the Genesis betrothal type scene (Genesis 29) in Jesus encounter with Mary and Martha outside the tomb of Lazarus and makes allusion to Song of Solomon 1:12 in John 12:3 and context. John again alludes to the Song of Solomon (3:1-4) in his depiction Mary's encounter with the resurrected Jesus in the garden (John 20:1-17).
Oneness. This theme makes sense of John's theme of oneness. The creation of Eve ends with man saying, “This is now bone of my bones and flesh of my flesh; she shall be called ‘woman,’ for she was taken out of man.” That is why a man leaves his father and mother and is united to his wife, and they become one flesh." "One flesh." The emphasis upon oneness between the Father and Jesus and Jesus and believers is stressed repeated throughout the gospel of John.