Short explanation: Lick the lens because I told you to.
Longer one: if the lens has a thin sheet of liquid on it, it's like looking through a thin sheet of glass; it doesn't distort the image. If you have a bunch of little droplets, it's like looking through a bunch of little beads, which does cause problems. Licking the lens puts a durable layer of liquid on the lens, so that any droplets that get splashed on there will quickly smooth out into a more uniform layer.
Of course, a totally dry lens would be even clearer. Rafters might be able to keep the camera high enough so that they don't have to worry about licking it. Licking the lens is for dealing with splashes.