Rolled specifically for us to restricted tolerances and hardness, stainless steel shim stock is very precise in thickness and shape.  Shims are typically used in order to support, adjust for better fit, or provide a level surface.   Over time, parts of machines can get worn away or ground down, resulting in the part losing its functionality, but the introduction of shims can reduce this wear and tear, allowing the machine to last longer and keep to its required levels of accuracy.