If you're referring to the fact that Nick doesn't give Gatsby's full story until Chapter 6, this information was withheld in order to present the persona of Jay Gatsby as one complete, albeit mysterious, individual. Nick gives the details of James Gatz when a New York reporter starts asking about Gatsby because of his notoriety and mysterious background.
Gatsby is introduced, in the novel, at the end of Chapter 1, although there is no dialogue between he and Nick. This is important because it establishes an image of Gatsby that is interesting and mysterious. When Nick spots him outside, he determines not to call out because it appears that Gatsby wanted to be alone and that he (Gatsby) looked to be trembling.
Involuntarily I glanced seaward — and distinguished nothing except a single green light, minute and far away, that might have been the end of a dock. When I looked once more for Gatsby he had vanished, and I was alone again in the unquiet darkness.
The reader is left with these images of the green light (symbolizing Daisy) and, adding to the mysteriousness of the man, the vanishing Gatsby.
Nick doesn't actually meet Gatsby until Chapter 3. Thus, Nick has had to, like the reader, wait a while before meeting him and this increases the mystery and uncertainty Nick has about Gatsby.
I would have accepted without question the information that Gatsby sprang from the swamps of Louisiana or from the lower East Side of New York. That was comprehensible. But young men didn’t — at least in my provincial inexperience I believed they didn’t — drift coolly out of nowhere and buy a palace on Long Island Sound.
In fact, no one, not even Jordan, is certain of where Gatsby came from. The late introduction is significant because even those who know Gatsby don't really know him enough to be certain of where he's from and who he is personally. And this is precisely because his persona (Gatsby) is made up. The late introduction increases the mystery of Gatsby as Nick continues to discover who Gatsby really is.