So, let's start at the foundation. Copyright law in the US is based on the US Constitution
Article I, Section 8, Clause 8 says:
The Congress shall have power...
To promote the Progress of Science and useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries
This clause covers both Copyright and Patent, the first for authors and the second for inventors, and the purpose is that by passing laws that guarantee that authors and inventors can profit from their creations for a certain time period, they will continue to create, and knowledge will be advanced.