It's high quality content that people want to share. Some of those people will inevitably share it on their blogs, with links pointing at the site. As a result, Yelp's pagerank improves and they stay ahead of their competition for things like "best restaurants in San Francisco".
A ton (I'd guess 80%+) of Yelp's traffic comes from Google / Bing search. Maintaining / growing that traffic is probably a substantial driver of their decisions.