It's beyond advice. It's the energy, community, contacts, drive, and exposure that you get to a community of like minded entrepreneurs. Getting _into_ that community is something that may change you for the rest of your life. Even just going for the _interview_ may be a life changing experience, regardless of whether you are funded.
In Silicon Valley, you see something similar in companies - there is this massive meritocracy and cross pollination that takes place. Companies get successful. Employees vest out and go form new companies. Then they reach back into their (metaphorical) Rolodex and pull colleagues from years passed who went through the most intense interview ever - working side-by-side for multiple years. It's why I recommend anyone from outside the valley interested in technology, particularly early in their career, to take any job they can get with a good valley company, and pay absolutely no attention to their salary. None. Once you are established, the money will follow.
But it's all about getting in the door, taking that first step. YC is one of those first steps for startup founders.