Vertically elongated pupils create astigmatic depth of field such that images of vertical contours nearer or farther than the distance to which the eye is focused are sharp, whereas images of horizontal contours at different distances are blurred. Species with horizontally elongated pupils are very likely to be prey and to have laterally placed eyes. Species with vertically elongated pupils are very likely to be ambush predators and active day and night. There is a striking correlation between terrestrial species’ pupil shape and ecological niche (that is, foraging mode and time of day they are active). In Why do animal eyes have pupils of different shapes? Banks, et. Eyes also tend to be forward set in predatory animals and more at the side of the head in prey animals. The pupils of herbivorous foragers tended to be mostly horizontally-elongated as is the case with goats. Ambush predators tend towards vertical pupils, where as active predators (those that chase down and kill their prey) tend more towards circular. The study distinguishes between herbivorous, active, and ambush foraging behaviors. This means that land animal pupils evolved as adaptations to the niche that they filled. There was a study published this last year (2015) that suggests that the pupil style of an animal is related to its strategy for predation. *- it's often stated that cats are nocturnal, this is untrue cats are crepuscular - that is they are most active at dawn and dusk. It is then easy to see why you get both kinds of pupil: circular pupils are favoured by animals that are typically active in bright light and need good vision under these circumstances vertical pupils are favoured by animals that are primarily active in low light but need some ability to see in bright light. Thus vertical slits let you be active in a wider range of light conditions at the cost of poorer vision in bright light whilst circular pupils continue to function well in brighter light but at the cost of not allowing such a large range of control over the amount of light entering the eye. So why have vertical pupils at all? Because, as you mention they're capable of letting a more controlled range of light into the eye. If you observe cats when they're hunting at dawn and dusk*, they have big, circular pupils it's only when they're in bright light that the pupil shrinks to a slit. Circular pupils are always functionally superior to vertical pupils a slit does not correctly focus light from all directions whereas a circular pupil does.

Cat pupil size meaning