Interesting! My pet theory about the crossover in the optic nerve is that it's the simplest way to get goal-seeking behaviour. Proto-eye activates, activates muscles on opposite side of body, organism turns towards activated proto-eye.
Fun speculation: Maybe we started with no crossover (which gives avoidance behaviour, keeping the organism free-swimming). This still works for a while as the axial angle between eyes and muscle groups increases, so there's no real penalty for having a bit of a twist. As the twist increases, it starts acting a bit like a discriminator, where we avoid small things less than large things, which seems good if we want to eat small things. Past 90°, we start spiraling towards things instead of away from them, which admittedly makes us crash into large things more, but we can chase moving things. Hunting has evolved!
Fun speculation: Maybe we started with no crossover (which gives avoidance behaviour, keeping the organism free-swimming). This still works for a while as the axial angle between eyes and muscle groups increases, so there's no real penalty for having a bit of a twist. As the twist increases, it starts acting a bit like a discriminator, where we avoid small things less than large things, which seems good if we want to eat small things. Past 90°, we start spiraling towards things instead of away from them, which admittedly makes us crash into large things more, but we can chase moving things. Hunting has evolved!