Just as a functioning intellectual property system can generate significant cultural and economic benefits, a robust public domain also contributes to a democratic society, a strong economy, and the advancement of science.
Public domain is different from "open access," which typically refers to works that are copyright-protected, but whose authors or publishers have chosen to make the work freely available to the public. Even if works are in the public domain, users should still acknowledge the source of the work, since failure to do so could constitute plagiarism.
The U.S. government, producer of the single largest public body of scientific and educational information, is one of the world's greatest contributors to the public domain. Its Office of Management and Budget's Circular A-130, "Management of Federal Information Resources," recognizes that government information is a valuable national resource and that the free flow of information between the government and the public is essential to a democratic society. U.S. government practices also have promoted broad dissemination of information generated by federal government funding. Grantees who receive federal government funding are strongly encouraged to share the results of their research.
[note: the above was taken from the U.S. Department of State publication, Focus on Intellectual Property Rights, which itself is in the public domain as a work of the U.S. federal government]