[theodorejb]

What benefit is an eye unless there is also the capability of processing and using the information? How would both evolve simultaneously?

[doctoboggan]

A photosensitive patch of cells could be wired directly to motor cells/muscles on the opposite side, which would allow the organism to swim toward the light (maybe useful for feeding or migrating, etc.)

[theodorejb]

How would the photosensitivity and wiring to muscles come about at the same time?

[jibal]

A fairly simple chemical reaction could cause an organism to turn or move toward or away from light in the ocean, with various imaginable benefits.

And note that box jellyfish have 24 eyes, some of them highly complex, but no brain. You can look into their behavior to find out what they do with the information.

[Azrael3000]

Stated clearly (0) has recently started a fantastic series about evolution that aims to explain bacterial flagella. It starts from basic principles and aims to answer questions like yours in evolutionary biology.

(0) https://m.youtube.com/watch?v=eFC9VzexRUk