It gives you freedom to customize your URLs and only your creativity sets the limit.
For instance, you can make your URLs shorter, as I do in my forum: from /threads/ to just /t/
Having shorther URLs is or used to be a SEO ranking factor.
Or if the language of your forum is not English, you can use the same feature to have parts of your URL in such other language: from /threads/ to /temas/ (into Spanish)
Having your URLs in your language is better for usability and could be also beneficial for SEO.
Again, those are just a couple of examples, but there are many, many more creative ways of using such feature.