I certainly am not claiming to be a Rockstar however I will answer your question in the best way I know how.

I believe in people truly adding value, I know from experience (knowing YC Alums) that YC adds value and can turn a good product into a great product. I am not too proud to know that there are lots of people out there who are Faster, Stronger, Smarter and probably better looking than me however I am also aware that if I am not actively seeking out those people (I believe YC has tons of them) then I am already a step behind those who are.

So while I truly believe with the right product and vision I will succeed no matter what I find no harm in trying to better myself via a program like YC.

I view YC as marketing. You gain exposure to your product/company/people via YC. You pay for the marketing with equity instead of cash.

I would say the same reason many established companies have. For the connections and learning that only YC can provide.

I wouldn't say anything YC provides can't be found elsewhere with some good old fashioned elbow grease and networking. But for a lot of people it will offer a big head start.

EDIT: What's with the downvotes? I thought HN was mostly hivemind-free, but I guess not when it comes to YC?