In part, they gain headlights into deal flow at an extremely early stage.

I can vouch for the Hacker Angels' willingness to help just for the sake of it, though. I pinged them when their announcement first hit HN, and while I am sure they were deluged, and my business is "pre-traction", they still took the time to respond in detail to my questions. This type of advice is difficult to get, especially from angels themselves. It's a great service to the community, and personally I don't mind them extracting a little value of out it!