At the risk of meta recursion - I write comments because they help me clarify my own thinking. This is the same reason I suppose people keep journals or write blogs or whatever. The points just provide a rough heuristic about how useful that particular thought was to other people. I don't always agree but generally the feedback is nice to keep me in check as I've discovered I'm not great at evaluating this myself. I read them because I like to hear other people's opinions.
Aside from this, I've used things I've learned reading HN articles and comments many times in my day to day work so I can't agree it provides no value. (I just learned about Boyer-Moore, for example)
I have to admit that sometimes HN comments feel like writing practice to me. I'm not aiming for internet points directly, rather the immediate feedback gives me a sense of how much my views resonate (or more importantly when they dont)
It's not clear to me, but people think I'm a very good writer. It could be it's because I write a lot to clients. But I think getting your opinion out without getting downvoted on HN has helped me dealing with clients and prospects to let them understand my opinion, in a non intrusive way.
Aside from this, I've used things I've learned reading HN articles and comments many times in my day to day work so I can't agree it provides no value. (I just learned about Boyer-Moore, for example)