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.
I think in this case, as Yelp have admitted in the past they are dependent on Google/organic search traffic, it is likely that this content is intended to function as SEO as well as (and due to the fact that it is) something cool.
With a company with more of a business-to-business angle, it might have been more ambiguous, but in this case it seems likely that SEO is part or all of the motivation.
