Wikis are mainly used to share data and collaborate with your players. When you set up a wiki, you can manage who gets to edit and view the wiki.
You can set up GM only
wikis as a place for campaign notes. Reminders of what is going on behind the scenes. If you're a player, it's a way to manage information that your character knows and that other players don't.
You can create wikis that everyone
in the campaign can see. These is usually for information that is common knowledge or that the party as a whole has learned.
Don't be afraid to make changes.
Old revisions of your wikis are kept and are accessible right over there -->
If you've just created a campaign, the wikis are a good place to post a lot of information that the characters should start with: rumors, histories, geographical data, important NPCs, etc.
This is also a good place to put "meta" information. House rules, phone numbers (and menus) for your gaming group's favorite restaurants.