Lighter pistons...rebalancing?

lighter is better on rebalancing price as you drill the crank counterweights instead of adding Mallory metal which is expensive. Lil cuda i could see that in a horizontally opposed motor but why do you balance cranks in 6s? Horizontally opposed i could see how the crank would weigh the same on both sides. why does the crank have counterweights? Not saying youre wrong just need a little explanation so i understand. i,m not a machinist so i like to know why things work.