If you add me to an existing discussion, you have to say why. Do you have a specific question for me? Do you want my opinion on something? Are you just sharing a funny joke?
Sometimes, I'll get a piece of mail that goes like this:
To: Aaaaa; Bbbbb; Ccccc; Raymond
--- Original Message ---
Gee, that's very nice of you to add me, but you didn't say why. Is this a FYI? Is there a question you want answered? Often, the discussion is just "Gosh, there's this bug, person A proposes a theory, person B proposes a counter-theory, person C runs some tests and has some preliminary results, adding Raymond."
It's like "Adding Raymond" is a ritual phrase people sprinkle into a mail thread. They don't know what'll happen when they say it, they don't even have any expectations, but it doesn't hurt to say it, right? "When in doubt, add Raymond."
If you don't explain why you added me to a thread, I'm just going to killfile it.