I see no reason to use additional mechanism. Why not use upvote? I'm upvoting stories so I could see them later (as ColinWright noted), why not create a page I could see comments I liked?
Separation of concerns. You might want to be able to tag a post or comment as noteworthy but not want to upvote it. For example, to remind you to come back to it and add a comment explaining why its wrong or that you disagree with it.
Perhaps to address a limitation in browsers wherein it is not currently possible to scope bookmarks to a given site during a search for said bookmarks?
Basically: Upvote != Save for Later