So many reasons for each!
Mom has a 17" MacBook Pro, mainly because she plays card games and appreciates the extra screen space for spreading out the cards.
I just bought a 15" MacBook Pro, because I take my machine with me a lot.
I edit video and graphics, and I almost always use an external LCD to expand my screen space, so my compromise is a smaller MBP for travel, with an external LCD for editing.
Those are just two tiny situations out of the millions of permutations of why someone would prefer one over the other.
Will this be your first MBP?