I don't like being waited on; it makes me uncomfortable, even when it's someone's job to do it, and I alleviate that discomfort by tipping them for it. When I put myself in that situation I feel like I'm being lazy ("I could pick this up myself, but instead I'm having someone do it for me"), and it feels appropriate that I should pay more for the privilege of being lazy. The tip is my way of saying "Sorry you're having to do this." It's silly, I know it is, but you asked, so there's your answer.