It helps to increase low-end torque. The theory is that the alternating bursts of pressure can help to pull exhaust away from the engine. "X" pipes are even better.
Back in the day we would even run a 1' to 3' section of 4" bus pipe off our header collectors to crate cooling vacume chamber then chokes down to size of exhaust! I know, there was no technology then. I miss those days!