Hebrew grammarians recognize what is called a cognate accusative. Ronald Williams in his Williams' Hebrew Syntax, 3rd ed. (Toronto: University of Toronto Press, 2010) defines this as:
When the accusative has the same root as the verb. (p.19, #51)
So in your case, the accusative is the noun "dream" (חֲלוֹם) and the verb is "dreamed" (חלם), with the verbs three radicals (ח ל ם) repeated in the noun.
However, that is where clear agreement ends. As Williams also notes in the same section:
A cognate accusative has been traditionally understood to amplify the verb (e.g., 'sheer terror' instead of 'dread'). Nevertheless, many cognate accusatives are unlikely to be emphatic ... and some scholars deny that a cognate accusative ever indicates emphasis.
So it may or may not be an indicator of emphasis. In the case of "dreamed a dream" it can be seen why Williams notes that many may not be emphatic, for it is challenging to grasp what emphasis would be intended in this case. Consider a couple of possibilities for English showing the emphasis:
- Deeply dreamed
- Intensely dreamed
Does such emphasis really add anything to the statement? I do not see that it would. What difference does it make (and therefore, why bring that out in the text) if one merely dreams, or instead deeply/intensely dreams?
So there may not be a clear grammatical reason, however ...
A search for the Hebrew verb חלם comes up with 29 hits, 8 times without reference to the "dream" itself, 19 times with reference to the "dream" in some form with it, and 2 times where the term does not mean dream (as the word has a homonym that has the idea of health/strength).
- "Dreamed" without the cognate accusative: Gen 28:12 (Jacob's ladder experience as it was occurring); Gen 41:1, 5 (account of Pharaoh's dream as it was occurring of the coming years of plenty and famine); Ps 126:1 (participle referring to one who dreams); Isa 29:8 (2 times; referring to hypothetical stories of a dream of a hungry and thirsty person); Jer 23:25 (2 times; referring to false prophets that are declaring they "generally" have dreams)
- "Dreamed a dream" (or similar close association of the cognate "dream"): Gen 37:5-10 (5 times, Joseph's telling of his prophetic dreams); Gen 40:5, 8 (butler and baker of Egypt telling of prophetic dreams); Gen 41:11 (2 times, butler recounting to Pharaoh he and the baker's dreams); Gen 41:15 (Pharaoh telling Joseph of his prophetic dream); Gen 42:9 (Joseph recalling the dreams of his youth when encountering his brothers again); Dt 13:1, 3, 5 (referring to one who guides/foretells by supposedly having prophetic dreams, but in these cases is not of God); Jdg 7:13 (a soldier in the Midianite/Amalekite camp recounting his prophetic dream that his fellow says is of Gideon's conquest to come); Jer 29:8 (reference to the false prophets being a cause of the supposed "dreams" they have); Dan 2:1, 3 (Reference to Nebuchadnezzar's prophetic dreams); Joel 2:20 (prophetic reference to young men having dreams after a future time of blessing from God)
- Homonyms: Job 39:4, Isa 38:16
In my survey of the uses above, it appears that the word is used:
Without the cognate "dream"
- whenever the narrative is referring directly to the event of the dream as it is occurring (such as Jacob or Pharaoh),
- whenever the narrative is referencing the concept of the action of "dreaming" in general (no specific dream being associated to it)
With the cognate "dream"
- sometimes when the narrative is referring to the dream being recounted after the fact (typically in first person by the dreamer) of its occurring,
- sometimes the focus is on the results of dreaming (i.e., often in the case of the false prophets' actions of having such so called "dreams")
But the simple noun form without the verb at all is used at times ("dream" is found 65 times in the text, only some of which are associated to the verb as noted above). These independent uses of the noun express either of the final two ideas as well. For example, just a reference to the "dream" (not the action of it) is in Gen 20:3, 6 in the recounting of Abimelech's warning dream from God, and in 31:10-11 of other dreams of Jacob's being recounted. Meanwhile, Num 12:6 speaks of the result of God having come to speak to a true prophet in a dream (the emphasis being on the fact that such a dream resulted in a communication from God).
Given that the verb can or cannot be included in some cases to express similar ideas, the implications are that the choice to say "he had a dream" (noun only) or "he dreamed a dream," or more commonly since it is first person, "I dreamed a dream" (verb with cognate noun) contextually puts some emphasis on the fact that the dreamer feels there is something significant about this dream, above other types of dreams he has had (whether truthfully in the case of those actually prophetic, or whether deceptively, in the case of false prophets trying to pass off their ideas).