2) To get out of your bubble. Being surrounded by other intelligent individuals all working toward demo day helps you stay focused and keep moving.
3) The experience. Being able to have gone through the YC system wouldn't hurt.
4) The seed money would be a huge help.
5) Access to VC's with the YC brand name behind you. So again, the money.
6) The alumni network. I hear it is extremely supportive even after YC.
Disclaimer: I applied to Summer '11
I am not saying these are good or bad things, I am just pointing them out.
My prior related remarks are here: http://news.ycombinator.com/item?id=1827795
I'm still hoping for two things: An infusion of cash to help me move forward (because getting well all came out of my pocket -- it wasn't covered by insurance and such -- so I have a financial mess impeding my life) and some means to rapidly make forward progress on learning a programming language/learning how to make a game. So far, nothing has really clicked. I think people think I am being ridiculous when I say things like "I would like to download a programming language to my brain". I'm not. I'm just trying to convey that I am well aware that some things just are superior methods and sometimes something just clicks with a person. (I'm "twice exceptional" -- smart but also handicapped. So under the right circumstances I can get impressive things done and under the wrong ones I look really incompetent.) I haven't found that something yet. Presumably, it wasn't yet the right time.
We are going to try another startup anyway -- and we realized just days before the deadline that YC was a possibility (though a bit of a long-shot). So we gave it a shot. It's just one of hundreds of things we'll try...