For a lot of hackers, the biggest benefit they get from YC is confidence. It happens almost instantaneously. You're no longer working on a project - you're starting a company. Advice, support network, and introductions are incredibly helpful, but they're nothing compared to confidence. If you already have it, you can certainly succeed without YC (although getting in helps). Theoretically, you can just tell yourself "I will be confident", in which case you can succeed without YC too. But for most people it doesn't work this way.

YC gives people the determination to persist against all odds - probably the most useful quality of all. It turns the fear of failure from working against you, to working for you. It's an incredible energy source. I would be very surprised if most successful YC companies did half as well without YC. Not because of the help that YC offers, but because of confidence and determination.