A complicated eye is useless without a complicated connection to a brain. An eye by itself is pointless. It has been claimed that this poses a problem for evolution.
The eye came first. In a fairly real sense, the brain is an outgrowth of the eye, and not the other way around.
Originally, an eye spot in a single celled creature just generated some chemical signal. Multicellular creatures eventually developed nerve cells, which connected sensors (such as an eye spot) to effectors such as cilia or contractile sheets. Later, there were true muscles, and eye cups, and some actual nervous wiring. After that, eyes and nervous system wiring could co-develop. But notice that a fish has quite good eyes, and not much of a brain. Clearly the original co-development mostly wired for simple behaviors and simple reactions. "Seeing," as we do it, involves understanding images. By that standard, fish have eyes but don't "see".