Locks to prevent simultaneous edits. A file lock isn't really needed until it's time to save changes. (Each previous version of an entry is appended to a history file, so an unauthorized edit can be undone.) A file lock is only necessary to avoid multiple saves from becoming confused (i.e., multiple instances of the Wiki software trying to write to the same file at once).
In other words, you don't lock a file as soon as someone reads its contents into an editor. Everybody can contribute, but only one person's edits will be displayed. (A more sophisticated Wiki might use Ajax to alert you if someone else is trying to edit the same page you're working on, but it won't tell the other person that he can't edit the file.)
