Question for YC Graduates

We often hear from the YC founders / partners that the biggest adavantage of YC is the YC community. But I never get to read what that really means on day-to-day basis of running a startup.

I can imagine some of those benefits. I've read that YC companies are often the first customers of other YC companies. I think I remember reading that there are is a forum/social network for the founders only (I apologize if I misremembered about this). I can imagine that if you identify yourself as a YC graduate and ask another YC-er for coffee you are unlikely to be turned down.

But what I would like to ask are some concrete examples of how this community works, and some concrete benefits that came out of being a member of the community.

If this question is inappropriate in any way, I apologize in advance.