I'll kick it off with my answer:
For me it is a number of factors:
1) Practice. As both a consultant and corporate warrior at different times C77 has given me space to hone my language, practice conveying points quickly an clearly, convincing others, acknowledging other's points. It has seriously helped me to navigate in my job better.
2) Recruiting. Seeing so many portfolios has been helpful to know the watermark out there. It is no surprise that the last 4 people I hired were all core77 posters. Most of the people I've interviewed either have logins or read the boards. More people just read along than you think. I interviewed someone from a large global automaker an he said they all read the boards but don't post because of their corp. policies. Anyway, having the opportunity to help people shape their portfolios has always been helpful.
3) Personality. I'm and older brother. I think joking around a bit while giving lots of advice is just part of my personality. I've always been more of a 2 way communicator than a one way communicator. Writing a blog post can be tedious to me. Writing 10 forum posts comes easy! I can do it in the nooks and crannies of time between meetings, or after I throw down a sketch as a break.
4) Community. There isn't much sense of a physical design community in the modern world we live in. I believe our profession absolutely needs a place were designers can be designers. Where we can talk about best practices, bat ideas around, and just nerd out. Having first started posting here when I worked for a 5 person firm, then a global footwear company, then a global strategic design firm, now a CE company, the boards have always connected me to designers outside my workplace and industry. It was key in making sure I never became overly focused on one industry or workplace.
5) Giving back. Now that I've gotten all of that from the boards, it would feel selfish to not give it to others in return. We help make this a place were the above 4 things happen. It frustrates me to see a promising student learn a bunch with some help from the boards, get a job, and move on without doing the same for others. Realistically though that is just reality. I try to be somewhat consistent so people know there are always a few people they can hope for a response from.