Boredom, keeping up with interesting news, seeing the opinions of others, contributing to the overall debate on something.
Other places you can comment on are things like YouTube or Reddit, but YouTube is kind of random and not any chance of a debate and Reddit is a left wing hell-hole.
It seems kind of standard these days to watch or read something online then comment on it. I actually find myself thinking of a question or comment to post while I watch Netflix, only to remember that it has no such feature.