With balancing, you equalize the weights of the small and big ends of the rods and all the pistons, then weigh everything that reciprocates and balance the whole mess by adding or removing weight to the counterweights of the crankshaft. If an engine is just a little out of balance, it can get on your nerves after a while. It might not be enough to cause any damage, but you know what I mean, it's a loose thread that needs to be handled. Balance it if you can afford it. You'll like the motor better in the long run.