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To prevent content theft for profit, most copyleft licenses state clearly that the acknowledgement of all preceding authors must be preserved in the work, and that a notice must be included in the work to indicate where it can be found for free. In addition, most copyleft licenses also require that all redistributed work include the original copyleft license so that anyone who receives a copy understand the copyleft concept.
Some copyleft licenses are ``viral''. This means that any derivation of copyleft work is automatically copyleft as well. This means if you take a copyleft document and makes changes to it, your modified version is automatically copyleft. You can keep it private for your own use with most copyleft licenses. However, as soon as you publish the document, it must conform with all the rules stated in the copyleft license.
The primary motivation of copyleft licenses is to make content (documents, music, video, program code and etc.) freely accessible to all so that the distributed work can be improved by anyone who wants to contribute. A prime example of this philosophy is Wikipedia (http://en.wikipedia.org. At this web site, documents (encyclopedia type) are authored, modified and maintained by anyone who wants to contribute.