It doesn't really generate activity. Basically any platform would prefer to have more comments to generate activity, which in some cases moves content across various algorithms, but also makes the platform look well active and involved. Having an active looking platform can make others want to sign up and join. I'm sure there's potentially other reasons.

That said, I think it's fine to lurk and should be promoted more. Sometimes it's better to spend time focusing on articulating your thoughts and writing your posts than it is just writing more posts overall, and sometimes there isn't much to add to a topic.