Their pupils are vertical slits so they can let more or less light in,

The same goes for round pupils so that doesn't explain why they are vertical. They are also mostly found in nocturnal predators, so regulating light intake isn't that biggest concern (in fact they often also have a tapetum lucidum to re-use the little light they can get).

From what I've read the vertical shape of the pupil is beneficial for judging distances (important for pouncing on things).