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Locking pages can be done to protect pages from the edits of non-admins. This is usually done for OGL pages, where we don't want anyone to edit them like normal pages (since the point is to preserve them in original form), but it can also be done for trustworthy users who wish for their complete pages to be locked, so that they can only be edited by admins. Please ask an admin for help with this.
If a page is being harmfully edited by anonymous users, you can request an admin to protect against unregistered users, allowing logged-in users to edit it, but preventing harmful spam or insistent edit warring. Please report such requests to an admin.