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You raise your base attack level when you constantly spam magika abilities in cities. It's a bug that's been here since pre .5 Alpha-Conga-SUPER!! build 87.
For the same reason people use /yell in cities. They get bored and think it is funny to grief others, and keep doing it until they can start an argument.
I think the general thought on the subject, according to the other posts, is that they do it to be annoying. I have to agree. Very fun to do it... Err I mean annoying when someone else does it next to the bank NPC or the crafting stations, since you still have the joy of seeing the pretty colors even though you are otherwise occupied.