House cats, like most other animals that have slit pupils, have multi-focal lenses in their eyes. Imagine a lens that has concentric rings with each ring focusing different colors. This gives the creature especially sharp color vision, even in dim light. The slit pupil lets light reach the different rings, even when the pupil contracts in bright light.

This arrangement functions best in animals with fairly small eyes, so slit pupils are rare in large animals such as lions and tigers. They have round pupils and single focus lenses. So do humans.