I asked this question in the cat section last week or so, but the answers weren't very good. Then I found out there is a zoology section, which is where I would've asked in the first place if I had known it existed.
Calimecita
Favorite Answer
Cats are nocturnal hunters. They have frontally placed eyes that provide good binocular (depth-sensitive) vision. In addition, their eyes are much more sensitive than ours to low light intensities, and they are able to see with very little environmental light (their minimum light detection threshold is up to 7 times less than that of humans). Due to this sensitivity, they could be subject to eye damage during the day, and their vertical pupils could be an adaptation to avoid this. The pupil is the "window" through which light reaches the retina, and a vertical pupil that can contract to a mere slit, in combination with the eyelids that close perpendicularly to this slit, is able to keep excessive light out. In contrast, the cat pupil expands to almost circular shape in the dark. They have other adaptations for night vision (see link).
Other crepuscular/nocturnal animals also have vertical pupils, e.g. many snakes and geckoes.
xxkiskekyxx
As far as I know cats eyes are shaped the way they are for the greatest infiltration of light. The slit shape allows a cats eye to expand or shrink much more greatly than out own eyes which accounts for a cat's amazing night vision.