It prevents backup. If the light goes from green to red on a major road, then arterial traffic stops (just like it is supposed to). Now, a crossroad dumps in a new batch of cars that backfill the next block. When the arterial light turns green, those cars now have nowhere to go. It’s not too long before you have cars blocking part of the intersection between light cycles out of frustration. Or, at least that is the case near where I live.