I was wondering if there's a reason to do so, other than to know if there's new activity.
I'll assume your talking about a situation where the number of comments is shown but the comments themselves are hidden (and clicking on the number of comments expands to view the entire thread)
People like to join into an active conversation, if they see an item with 10+ comments (for example) then they are probably going to assume the original post (or the comments) have something interesting, exciting, or controversial going on.
If you previously commented on an item then it could be an indicator of if new activity has happened since then but that's not the main utility in my opinion.
In certain situations I would also say the number of comments can aid the credibility. If you go to a blog for example and see 0 comments on anything you're not going to comment yourself most likely because it's a deadzone -- and that's why major blogs are going to tout every chance they get how many comments are going on in a particularly active thread.