Because I feel like I have something to add to the conversation. A data point, a related story, a rebuttal, a joke (on reddit), etc. I often find comments to be

Because I feel like I have something to add to the conversation. A data point, a related story, a rebuttal, a joke (on reddit), etc. I often find comments to be as good or better than the content they are about (If and only if it's a well moderated subreddit or HN) so when I can contribute I like to do so. Seeing people's different takes forces me to think more about an article/blog post than I otherwise would and sometimes challenges my own "beliefs"/thoughts which I think is a good thing. As for HN replies I think a lot of people use a service like this: https://www.hnreplies.com/ https://www.hnreplies.com/ to get notified, it's what I use. Sometimes it does feel like screaming (or whispering) into the void if it's a post with a ton of comments on it already, though often if there are that many comments then my viewpoint is already represented so I just upvote that instead or expand on it if I have something to add. Rarely will I post a top-level comment if the thread (on HN or Reddit) is already large since I know it will get lost in the shuffle. You say posts have a short life and that's kind of true but not always. It's less the case with Hacker News (though I do enjoy perusing the "previous discussions" of a link that's reposted years later) but Reddit (rightfully) comes in most search search results for things I'm looking for. The top example being video games for me. I can't tell you how many 7-10+ year old threads I've read to find answers to the questions I have. I've even commented on some of those "ancient" threads to give an update or "how it works today" for the next person that finds that post via search/google. Not all content is evergreen but a lot of it is. Home improvement, cooking, and city-specific questions are some more examples of things I've found years after the fact on Reddit that were exactly what I was looking for.