Unless a site is massively popular with lots of comments on nearly every post, leaving a comment on a post can feel a little lonely.
Single posts tend to feel a little walled off from everything else. Once you leave a comment on a post, you may feel like browsing around and finding another post to read, but you don’t typically get the impression that you’re in the middle of thriving discussion.
That’s where forums can excel.
Although a forum thread is also walled off to degree, you’re always only one click away from lots more discussion.