moon_aka_sun

1. Evolution gave cats this kind of pupil to compensate for the excessive entrance of light in the eyes. The cat has an elevated visual capacity in the dark, its eyes are therefore very sensitive to light. The eye must have a good control on the quantity of light that it lets enter during the day to avoid damage to the retina . The combination of a vertical pupil and horizontal lid gives them a certain advantage and therefore is very important for the cat's vision. See also here.

2. The answer for the vertical slit is that it improves an animal's focus in the direction perpendicular to the slit. Thus, cats and snakes hunting close to the ground can better detect their prey over a wide horizontal field. <...> But puzzles remain, otherwise we wouldn't address this subject. (1) Some snakes have round pupils having apparently been subjected to different environmental forces; (2) Of all the 9,000+ species of birds, only the skimmers own pupils with vertical slits. You would expect ground-feeding birds like robins and larks to also have them; (3) Most interesting are the cuttlefish with pupils shaped like Ws. That's a tough one. See also here.

3. The reason that nocturnal hunters tend to have vertical pupils while diurnal (daytime) hunters tend to have round ones, has to do with the ability of the pupil to adjust its opening quickly. For nocturnal hunters, the pupil must open very wide to gather all available light (same reason they have reflective backs), yet they need to be able to shut down very tight in bright light. A diurnal hunter, in contrast, never really is asked to use its best vision at night, and thus does not have as much need to adjust pupil size. As to why vertical slits are easier to adjust than round pupils, it's only a guess, but I think it has to do with the mechanical forces of the muscles involved; its just easier to build a rapidly adjustable slit than a rapidly adjustable round pupil.

Etc, etc, etc.

Also interesting: Vision In Dogs, Horses, and Cats; Snellen chart (in wikipedia too).