I suspect comment sections outside of forums (or sites which are designed around discussion like HN or Reddit) are mostly attempts to make a site more "viral" and "social", in the belief that doing so will somehow drive people to your site and keep them engaged and maybe clicking on the ads or whatever.

While this could be true, with certain sites (like news sites) comments are bound to be politically charged and tend towards the negative. Doubly so if comments allow threading or reflinks, making tangential discussions (and arguments) between posters easy. I think many sites assume comments would somehow lead to a revenue stream because something something facebook.

Also, it's probably likely that any site built on Wordpress is going to keep comments by default in their templates.