Mainly just to get the beer of the trub (hops, break, old dead yeast cells) to prevent any off flavors that may cause.
Funny, I recently heard a TBN broadcast denouncing secondary ferments.
While I don't disagree with the logic, I do believe in some kind of secondary ferment. I happen to do mine 9 times out of 10 in a corny keg. I find it easier to dry hop and I do think my beer is clearer and more visually appealing after a short secondary.
Is it essential to good beer, probably not.