They also offer a number of programs/perks, like workshops on various aspects of business development, networking events, speakers, negotiated rates with lawyers and other services, classes on entrepreneurship, and access to grants and interest free loans. With their guidance I was able to win money in some business plan competitions and drop to part time in order to fully pursue the business. Recently they have helped us find a few angel investors as well.
Without them I'm not sure where I would be. I started out as a single founder and now I'm to the stage where I can bring on three or four friends/employees full time to help me take things to the next level.
Perhaps the best part is the constant ability to both collaborate and give back. There are opportunities everywhere for bouncing ideas around and helping other startups with their problems.