"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;" (U.S. Const. art. 1, § 8)
The goal of the copyright clause in the U.S. Constitution was to help and encourage authors, scientists, inventors and other artists to continue their creative endeavors. To do this the U.S. Constitution calls for those creators to have an exclusive right to their works for a limited time.
The rationale being, that if creators received a direct benefit from their work in their lifetime, this would help them continue creating new works and making new discoveries. After enough time elapsed for those creators to benefit sufficiently, those works would pass in to the public domain, where all members of society would be able to access, study, question and possibly build upon those works. This process was considered to be an important part of developing a rich and active intellectual life in the culture of the new nation.
The copyright landscape has changed significantly since 1787, with author’s transferring their rights to publishers, corporate institutions or other individuals. These actions add another layer of complexity to copyright issues. Also the U.S. has passed other laws to help bring copyright standards in line with those of the international community. Due to its currently complexity, copyright questions are often approached with anxiety. However, it is helpful to keep in mind that the original goal of copyright law was to encourage intellectual endeavors, not hinder them!