When a wheel is originally fitted to a vehicle it must be perfectly balanced. Over time, wheels lost balance resulting in the distribution of weight and contact patch (that part of the tyre in contact with the road) to differ.
Wheel balancing was created to set the weight to distribute equally again and ensure the tyre rotates evenly. To achieve this, small balancing weights are added to the rim to counter weight inconsistencies.
Visualize a weight is added onto one side of a set of balancing scales – they’ll certainly be heavily unbalanced. However, if you start to add lighter items to the opposite side, the scales will begin to even out until the original weight is completely counterbalanced; unless you add lots of smaller items, causing the scales to tip a different way.