The plants have evolved with the ability to make food from solar energy, and the sun doesn't stand still. You will get a healthier plant with a moving light source, because it exposes more surface area of the plant to light, instead of just having the top of the plants doing all the work.

Experiments done with pea vines on a turning wagon wheel in the eighteen hundreds showed that they grew towards the center of the wheel. The wheel was mounted horizontally, and the sunlight was from above. They believed this proved that plants did not just turn towards the sunlight, but actually grew away from the center of the earth against gravity. That is why a carpenter's level held to line up with the trunk of an average mature douglas fir tree in the distance, will show it to be perfectly plumb.