[Tommy]

Let me try a slightly less technical explanation. When you spin an object, it will always try to spin about its center of mass. For example, if you spin a childs toy top, it will spin about a line passing through its center of mass. If you spin a crankshaft assembly, it naturally wants to spin about a line through its center of mass. If the assembly is not balanced so the center of mass aligns with an imaginary line through the centers of the bearings that hold the crankshaft, the shaft will constantly be pushing against the bearings. As an unbalanced crankshaft rotates, the magnitude of the push on the bearings increases as the speed of rotation increases. So, balancing the rotating assembly should increase the life of the main bearings and the crankshaft, especially in engines run at high speeds.