Here’s the thing, it is weird for today technology, because you never truly go away from the chatroom. But back in the IRC times joining a chatroom was very similar to entering a room, you would only see messages sending from then on, so if you wanted to keep track of a chat you were having after you left you needed to leave your computer connected and online, so it was impossible to know when someone was online and when they just had left the computer on to follow up on a thread that was happening when he went to sleep, so it was a common courtesy, just like saying hello when walking into a room. It was a way of telling people “I’am here now”, but most chatrooms today have an away status you can set.

I agree it doesn’t make sense on discord or whatever, which is why I don’t do it there. But it might be one of those things that people just keep perpetuating because it’s what they always did.