And, really, what is Facebook but Internet commenting on baby and cat photos of people you are related to in some form (by choice or by birth).
If we accept that humans are social creatures, and social media is able to connect people, then it is possible to say Internet comment sections are able to fulfill some needs that people have. Displaced football fans working far from home may not be able to find a local group that also supports their home team in a far away place, but there's a digital clubhouse for the displaced fan's home team somewhere on the Internet.
This may not satisfy a need like food or water, but people organized into geographically-based, sometimes tightly-knit communities before the Internet. That modern technology allows us to organize into loose-knit communities through the use of "Internet comment sections" is to realize J.C.R. Licklider's dream of an inter-networked computer system.
People (at least some people) tend toward the need for discourse to temper and test their ideas. I mean, you're commenting on HN.
It's the guidelines. By writing guidelines that promote useful discourse you allow everyone involved to learn more about their own biases and shortcomings.
Every person that writes their opinion grows when reading the varied responses (as long as they're open to other people's perspectives).
That's pretty much it. Once the population gets too big and the level of discourse too low, the intelligent people will move to greener pastures with better signal:noise ratios. Slashdot's moderation system didn't save them, and HN's moderation system won't either.
I've yet to see a comment system that promotes signal:noise ratio over groupthink.
Doesn't every groupthink community describe themselves as "Smarter and more insightful commentators"? I'm pretty sure Trump or Clinton voter websites consider their signal-to-noise high and everyone else's low.