From the "Philosophy" section of ycombinator.com/about (the whole section is relevant really): One concrete consequence is that Y Combinator funding lets you s

From the "Philosophy" section of ycombinator.com/about (the whole section is relevant really): One concrete consequence is that Y Combinator funding lets you sell early, if you want to. It can sometimes make sense to sell yourself when you're small for a few million, rather than take more funding and roll the dice again. Google likes to do early-stage acquisitions, and we expect them to become increasingly common as other companies learn what Google has. If you take a large amount of money from an investor, you usually give up this option. But we realize (having been there) that an early offer from an acquirer can be very tempting for a group of young hackers. So if you want to sell early, that's ok. We'd make more if you went for an IPO, but we're not going to force anyone to do anything they don't want to. I think YC prefers big successes but doesn't try to pressure everyone to be the next Dropbox or AirBnB