Yeah but they wear it on their backs... It might help them when running away from archers. Other than that, the only thing I can see this does is make it harder for the archer to aim. On the other hand, it hardly mattered if they could make out the individual man because they never aimed for the individual man but rather the entire unit.

I'm thinking it is just a ways for soldiers/warriors to identify their leaders, which probably makes leading an army easier.