I had never heard of Y Combinator until Hacker News. As far as brand building goes, most investment companies could only dream of Hacker News.

The HN jobs section attracts candidates (visitors of HN mostly are devs) for startup funded by Y combinator, I guess this is a win-win

I think a lot of Y Combinator companies get preferential treatment for posts (especially job listings). So in that sense HN does have ads.

may be indirect benefits