Wikis are often par of the free culture movement. Free content licenses allow anyone to reuse, modify, or distribute the content free of charge for any purpose as long as they adhere to the license. For example, they must attribute the original authors and keep the content under the same free license even if they change the content.
The reason that many good wikis use this is that it allows the creators of the content to maintain their copyright on their own creations whilst still allowing other people on the wiki to make changes to it. It also means that the community is free to copy their content to another site if they want to, which wouldn't be allowed on the sorts of sites that claim they own the copyright to the content that you add.
See also What license does Wikianswers use?