If someone infringes on your copyright, it is extremely difficult to show the amount of harm you suffered. Actual damages, such as lost profits, would need to be documented in some way, and for many works this is next to impossible.
U.S. copyright law could allow you to recover statutory damages instead of having to prove the actual losses that resulted from infringement. Statutory damages relieve you of the burden of proving actual damages by allowing the recovery of a pre-determined award. You must meet strict rules regarding the timing of registration in order to recover statutory damages.