Here’s what Daniel Dennett has to say about vision in Darwin’s Dangerous Idea:
We know that eyes have evolved independently many times, but vision is certainly not a necessity on Earth, since plants get along fine without it. A strong case can be made, however, that if an organism is going to further its metabolic projects by locomoting, and if the medium in which the locomoting takes place is transparent or translucent and amply supplied with ambient light, then since locomoting works much better (at furthering self-protective, metabolic and reproductive aims) if the mover is guided by information about distal objects, and since such information can be garnered in a high-fidelity, low-cost fashion by vision, vision is a very good bet. So we would not be surprised if locomoting organisms on other planets (with transparent atmospheres) had eyes. Eyes are an obviously good solution to a very general problem that would often be encountered by moving metabolizers. (p. 128)
Dennett makes a pragmatic case for the evolution of vision — locomoting “works much better” with eyes. The pragmatic value of visual acuity can be understood only to the extent that the visual system accurately conveys information to the organism about the “ambient optic array” (JJ Gibson’s term) generated by the environment through which the organism locomotes.
It’s likely that, beyond a certain level of visual acuity, the costs of improved vision (more retinal cells, more refined neural processing of retinally-conveyed information, etc.) don’t outweigh the benefits (“at furthering self-protective, metabolic and reproductive aims”). So, for example, humans don’t need as much visual acuity as hawks because humans aren’t as likely to increase their chances of survival by spotting small prey animals at great distances. However, if an individual member of a species has less visual acuity than its conspecifics, then that individual will likely find itself at an adaptive disadvantage, since the visually-impaired individual is not equipped genetically to redeploy bioresources usually allocated for vision in order to compensate for its visual defects. In short, the visually-impaired individual doesn’t just see differently from its peers; it has poorer vision, both informationally and pragmatically. I would recommend that this creature be fitted with corrective lenses.
This all seems pretty obvious, doesn’t it?