παραβολή is almost always translated as "parable" and, with 2 exceptions, is used only in Matthew, Mark, and Luke (never in John). The OP's question shows there is a potential for ambiguity: is the purpose to teach those slow to learn to to confuse someone who is listening?
A common Biblical definition of parable is "an earthly story with a heavenly meaning." The original meaning is "a placing of one thing by the side of another, juxtaposition, as of ships in battle, Polybius 15, 2, 13; Diodorus 14, 60."
1 Just like the two ships, the two meanings of the parable may be considered "side-by-side," and the use of a parable will divide listeners:
One ship | Second ship
Earthly story | Heavenly meaning
Slow to understand | To be confused
To accurately juxtapose two ships an external marker is needed. Similarly, the two components of a parable may be used to divide the two types of listeners:
North Heavenly Meaning
One ship | Second ship Slow to understand | To be confused
South Earthly Story
The same story which is used to teach one who is slow to understand (Matthew) can be used to confuse (Mark). One slow to understand will benefit from an earthly example which illustrates the meaning. Confusion, on the other hand, is not as obvious. It could be on the story or meaning; it could be the decision to teach using a parable. For example, a scribe or Pharisee could be confused over Jesus' use of an "inappropriate" method. So if a parable was used purposefully to confuse, the confusion could be over either the methodology or the message.
Despite the aspect of confusion, there are two primary factors on the necessity of parables:
- Are those "slow to understand" in attendance?
- Does a teacher want to reach these?
The negative impact (to confuse or be confusing) could be intentional (as Mark seems to say), or simply an unavoidable consequence of trying to reach the "slow to understand."
Matthew and Mark
Jesus' teaching as presented in Matthew can be divided into three main methods:
To everyone without parables | Using parables | To the disciples without parables
Up to Chapter 12 | Chapters 13-23 | From Chapter 24
The decision to use parables begins after Chapter 12 which describes how the Pharisees conspired to kill Him (Matthew 12:14); how the scribes and Pharisees accused Him of casting out demons by Beelzebub (Matthew 12:22-32), and how the scribes and Pharisees asked for a sign (Matthew 38-42). If the decision to use parables was intentional to confuse, it was done in response to the actions of the scribes and Pharisees.
Mark also places the first use of parables as a response to scribes making an accusation:
22 And the scribes who came down from Jerusalem were saying, “He is possessed by Beelzebul,” and “by the prince of demons he casts out the demons.” 23 And he called them to him and said to them in parables, “How can Satan cast out Satan? 24 If a kingdom is divided against itself, that kingdom cannot stand. 25 And if a house is divided against itself, that house will not be able to stand. 26 And if Satan has risen up against himself and is divided, he cannot stand, but is coming to an end. 27 But no one can enter a strong man's house and plunder his goods, unless he first binds the strong man. Then indeed he may plunder his house. (Mark 3)
Both Matthew and Mark place the first use of parables after the same (or similar) incident: accusing Jesus of being possessed by Beelzebub. It is this accusation which initiates Jesus' use of parables when teaching in public.
We can say the parables which are necessary to reach those who are slow to learn, were also purposeful to confuse those who believe Jesus is possessed by Beelzebub. The "outsiders" are those who believe Jesus is from Beelzebub whom Jesus purposely tries to confuse (ironically) by teaching a heavenly message. In this case, the earthly example illustrates both a heavenly message and a heavenly messenger.
1. Thayer's Greek Lexicon