Tommy has stated it really well without all of the technical jargon. If the top he used as an example were out of balance or has just a little more weight on one place than another you would get wobble. If it is balanced it spins smoothly. An out of balance crankshaft will tent to try and wobble if that is the correct term and that causes uneven stress on the whole shaft.
Besides that it makes it easier to carry the engine with the weight even.
Ron
