The classic explanation for this is that it reduces visibility under ordinary topside illumination. However, the dynamic effects are much greater. When the flock shifts orientation it is very difficult to follow a single target against a background that closely matches the bird's tone. By shifting back and forth in tone the flock can make it very difficult for the predator to track a single target long enough to line up for a kill. I have seen it almost disappear in a flash.
Predators usually have to hit a single target to make a kill.
By making it hard to track any member, a flock can increase the survivability of all of its members.