Having a cell that senses the direction from which light is coming would certainly give the organism an advantage. It’s basically simple, mutations which benefit survival are are retained. Take a fish that has leg like fins and a proto lung can get away from predators limited to the water.
"simple mutations which benefit survival" would have to start out as a single microscopic mutation of a cell. Then, continue mutating in a fashion that would eventually be a benefit - a "happy accident".
It seems to me that a mutation within a given species which initially serves no purpose - would disappear.
Are suggesting that "happy accidents" are the primary driver of most of evolutionary gains?