[andyjohnson0]

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.

Basically: Upvote != Save for Later

[icebraining]

Isn't that what bookmarks are for? Why do we need HN to recreate basic browser functionality?

[politician]

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?

[icebraining]

That's what folders are for. And in any case, if you bookmark the comments page, you can then search by the hn domain, at least on Firefox.