Wikis are an on-line format for collaborative vandalism. They'll tell you that it's for the creation of articles on a subject.
The Truth Edit
The Truth is that wikis are a vandalism-themed MMORPG. Each wiki comes preinstalled with hundreds of perfectly-formatted pages of content, and it is up to the people of the internet to vandalize every page until it is unrecognizable. On some wiki hosts, most notably Wikia, players are given achievement points for an increasing amount of vandalism.