Why would one put tennis balls in the spokes? I see this once in a great while around the city: two or more tennis balls stuck inside the spokes, right up against the hub. It usually seems to be on older-looking mountain bike commuter/beater bikes, but I think I saw it on a newer fixie a while back, too.

Does this server some function to which I am not privy?
I believe it's a stylistic choice, and doesn't actually serve any function.