Take a look at the case of Sony v. Universal City Studios:

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 and inventors 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. (p. 429).

Now let's break this dictum down: The purpose of copyright and patent is (1) to motivate authors and (2) to allow the public access after the limited period of exclusive control has expired.

In other words the purpose is (1) to encourage authors to (2) enlarge the public domain.

In other words, the public domain is where a work rightly belongs; copyright and patent are temporary expedients designed to move the work into the public domain.

Here's a patent-law case: Bonito Boats v. Thunder Craft Boats:

The ultimate goal of the patent system is to bring new designs and technologies into the public domain through disclosure. (p. 151).

If the public domain is not an appropriate place for "new designs and technologies", then why make a law to bring them there ?

T.