I can see some evolutionary advantages for a predatory animal like the cat in being able to subitize[1] (I suppose cats are also small enough to be prey themselves historically, so just as useful to count threats as opportunities). My cat like to turn on my bathroom faucet to play with the water, and will often unscrew the drain stopper once he's bored with that. I have to keep the drains closed and wire ties on the taps...