Comments help fixing issues in a decentralized way, which is awesome. They also help develop stickiness, like HN (or addiction if you fall into dopamine addiction). Youtube would never be popular without the comments, which help criticize the video and get other viewpoints.

But it seems they aren’t valuable _enough_ that someone would develop a browser extension to comment on any webpage —or, since such extensions have been tried a lot actually, that people would actually use them.