The key to a WIKI is collaborating... it is a great way for students to work on projects with the guidance of a moderator (teacher). I can see a WIKI being used by students from different schools collaborating on a cross state project like the 3rd grade Virtual Museum project. Below is a quotation from my searching for a WIKI. I actually did not find a WIKI from a primary classroom which I liked or understood.
Wikis are used in the “real world” (outside of K-12 schools) by people collaborating on projects or trying to share things online, such as family information and photos, technical information from users of a product, data from a research and development project, wine expertise, travel journals from abroad, club or specialty information, or projects like collaborative cookbooks.