What's the purpose of having an history ?
The goal is to be able to track the changes that happen to a page and also to prevent vandalism. If a bad person wants to clear all the page and the page is not write protected, it can do so. But as anybody has access to the history of the page, anybody can restore the page to its previous state.
This can also be usefull if you want to make some diff between the pages to see what has changed between several edition. For example, let's say you contributed a page and several persons modify it in the same day. Perhaps would you like to see exactly what change have been made. In that case, you can use the history and make a diff between your version and the latest one to see exactly what has been changed.
Accessing the history
Click on the history link in the edit area.
You will be shown the history page.