Cats have vertical pupils as it helps them to hunt.
As given in the site Why cats have vertival pupils
Ambush predators, like many cats and snakes, were most likely to sport vertical-slit pupils, particularly when those animals were active at night. The reason for this correlation most likely has to do with the mechanics of the eye, Banks told Live Science. Ambush hunters need to be very good at gauging depth so they can effectively leap out at their prey.
There are two ways to gauge depth without moving. In one method, stereopsis, the brain compares the distance between the two images returned by each eye to gauge depth. (Hold out your finger, focus on it and close each eye in succession. That "jump" you see is the distance used in stereopsis.)
The other method, blur, takes advantage of the fuzziness of objects behind and in front of the spot on which you're focusing.
As it turns out, the side-to-side displacement used in stereopsis is easier to gauge using vertical lines and contours than it is with horizontal ones. Thus, Banks said, the vertical pupil provides the best view for stereopsis.
To judge horizontal distances, though, cats and other slit-pupil predators likely use blur, Banks said. To maximize blur, the pupil must open wide. And to maximize blur for horizontal lines, the pupil must open wide from top to bottom. In other words, the ideal shape is narrow horizontally and wide vertically — precisely the arrangement of a cat's eye.
"This is the right arrangement to maximize stereo and blur as cues to distance simultaneously," Banks said.
I guess that since your cat is mostly an indoor cat, who is hand feeded by you, and never had any experience of hunting, with time gradually her pupils have become more like human pupils.
Predators who are active at night have vertical pupils.
Predators who are tall and active at both night and day (eg. humans) have horizontal pupils.
