The Great Gatsby: Nick the Narrator

It is very clear that Nick Carraway is recounting all of these events and is supposedly the author of this story (it's really F. Scott Fitzgerald of course); however, what is not clear is his reason for writing all of these events.  The plot is interesting and all, but I don't know the purpose for why he would write all of this.  And in the very beginning of the novel when he was introducing himself with his background and a little bit of his past, it seemed like he definitely had a purpose for writing.  I'm just getting a little lost as he elaborates about seemingly pointless things, such as the really confusing part about the car crash after the party.  I'm just not sure why that is at all important.  Anyways, it seems like he has a point, like when he says "Reading over what I have writhed so far, I see I have given the impression that the events of three nights several weeks apart were all that absorbed me" (Fitzgerald, 55).  The simple fact that Nick cares about his writing so much makes me think that there is some greater picture, I'm just not sure what it is.  Hopefully I'm not overlooking something really simple.