The Constitution of the United States provides that Congress has the 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." The framers of the Constitution did not discuss this clause at any length prior to or after its adoption. The purpose of the clause was described in the Federalist Papers by James Madison:
The Constitution outlines both the goal that Congress may try to achieve (to promote the progress of science and useful arts) and the means by which they may accomplish it (by securing for limited times to authors and inventors the exclusive right to their respective writings and discoveries). The Supreme Court has often spoken about the purpose of copyright:
We have often recognized the monopoly privileges that Congress has authorized, while "intended to motivate the creative activity of authors and inventors by the provision of a special reward," are limited in nature and must ultimately serve the public good.
The primary objective of copyright is not to reward the labor of authors, but "[t]o promote the Progress of Science and useful Arts." To this end, copyright assures authors the right in their original expression, but encourages others to build freely upon the ideas and information conveyed by a work.
The economic philosophy behind the [Constitutional] clause . . . is the conviction that encouragement of individual effort by personal gain is the best way to advance the public welfare through the talents of authors and inventors ... Sacrificial days devoted to such creative activities deserve rewards commensurate with the services rendered.
The monopoly privileges that Congress may authorize are neither unlimited nor primarily designed to provide a special private benefit. Rather, the limited grant is a means by which an important public purpose may be achieved. It is intended to motivate the creative activity of authors . . . by the provision of a special reward, and to allow the public access to the products of their genius after the limited period of exclusive control has expired.
[C]opyright is intended to increase and not to impede the harvest of knowledge ... [T]he scheme established by the Copyright Act . . . foster[s] the original works that provide the seed and substance of this harvest. The rights conferred by copyright are designed to assure contributors to the store of knowledge a fair return for their labors.
The copyright law, like the patent statutes, makes reward to the owner a secondary consideration ... It is said that reward to the author or artist serves to induce release to the public of the products of his creative genius.
Copyright is "intended definitely to grant valuable, enforceable rights to authors ... 'to afford greater encouragement to the production of literary works of lasting benefit to the world.'" The purpose is not to reward the author, but the law does so to achieve its ultimate purpose -- "to induce release to the public of the products of his creative genius." The "immediate effect" of the copyright law is that authors receive a "fair return for [their] creative labor"; however, the "ultimate aim is, by this incentive, to stimulate artistic creativity for the general public good."
Congress also interpreted the clause when it enacted the Copyright Act of 1909:
The enactment of copyright legislation by Congress under the terms of the Constitution is not based upon any natural right that the author has in his writings, ... but upon the ground that the welfare of the public will be served and progress of science and useful arts will be promoted by securing to authors for limited periods the exclusive rights to their writings ... 
By granting authors exclusive rights, the authors receive the benefit of economic rewards and the public receives the benefit of literature, music and other creative works that might not otherwise be created or disseminated. The public also benefits from the limited scope and duration of the rights granted. The free flow of ideas is promoted by the denial of protection for facts and ideas. The granting of exclusive rights to the author "does not preclude others from using the ideas or information revealed by the author's work."
While copyright law "ultimately serves the purpose of enriching the general public through access to creative works," copyright law imposes no obligation upon copyright owners to make their works available. While it is hoped that the potential economic benefits to doing so will induce them, copyright owners are not obligated to provide access to their works -- either during the term of protection or after. Hence, unpublished works never distributed to the public are granted as much (if not more) protection as published works. However, once an author publishes a work, copies of the work must be deposited with the Library of Congress for the benefit of the public.