the idea is that the balancer shafts cancel out the vibrations from the crank / pistons, thus you get a nice smooth tickover.
I believe the main reason for them is to reduce emissions, the lower the tickover the less emissions but the more the engine vibrates.
in reality they do little, especially if you have tuned the engine.
certainly after fitting performance cams the tickover is smoother if you also fit a balancer delete kit
GM have stated that each balancer shaft requires 5 bhp to turn, plus the engine response is much improved when they have been deleted.
