Why do cats' eyes contract to a vertical slit?

Reducing the pupils to slits, rather than tiny circles, gives the cat a more refined control over precisely how much light enters the eyes.

For an animal with eyes sensitive enough to see in very dim light it is important not to be dazzled by bright sunlight, and the narrowing of the pupils to tight slits gives a greater and more accurate ability to cut down the light input. The reason why cats have vertical slits rather than horizontal ones is that they can use the closing of the lids to reduce the light input even further. With these two slits the vertical one of the pupil and the horizontal one of the eyelids working at right angles to one another, the feline eye has the possibility of making the most delicate adjustment of any animal, when faced with what would otherwise be a blinding light.

Confirmation of the fact that it is the nocturnal sensitivity of the cat's eyes that is linked with the contraction of the pupils to slits, is found in the observation that lions, which are daytime killers, have eyes that contract, like ours, to circular pinpricks.