Copyright is intended to protect you, the author of the work (personal or moral rights) and to stimulate the distribution of your work (exploitation rights).
Historically, publishers have taken on the part of the copyright dealing with exploitation (publishing and multiplying the work), because of this eventually they started claiming the exploitation rights completely and exclusively. In contracts the author had to exclusively transfer these rights to the publisher. Due to developments in the scholarly community (developments such as the internet and the Open Access movement) many publishers no longer claim the exploitation rights exclusively, but they agree to merely execute these rights on behalf of the author. This agreement is usually reached in a non-exclusive licence.
Publishers can still do whatever they want with such a non-exclusive licence, but ordinarily they do enforce an embargo period before other use can be made of the publication. During this period the author cannot multiply and/or make his work public in any other way, such as submitting it to a institutional repository. This is the period in which the publisher can earn back money from his investment (coordination of peer review / editors / distribution / promotion).