It's for "discussing" things. Doesn't have to be world-changing, or things that happen to be fascinating to everyone, or even associated with the original topic in the original post on the thread.
Just discussing. How many miles we get out of a tire seems to be a reasonable discussion topic for a thread entitled "Tyre wear".
But it's also true that sarcastic, mean, or just plain silly posts are ALSO included under the heading of "discussion", so you're all right. Carry on!