The greatest benefit of adding topics to posts is that it allows posts to be distributed in feeds to people who follow a specific topic. This has become all the more necessary, as Quora in its wisdom has removed the ability to assign a topic to an entire blog.
Of course, Quora has severely reduced the visibility of posts in feeds, and unless you actually subscribe to a blog, chances are you won’t see any posts in your feed anyway. I have seen posts in my feed only rarely, and I’m not completely sure that wasn’t a bug.
Topics do appear as a filter in the search page, so they can be used to retrieve posts in search. I doubt the topic filter functionality is used all that much though.
Likewise, you can browse all content associated with a topic, and that will bring up posts as well as answers. I suspect that functionality is used even less.