The basis for a functioning door lock: no door cylinder can be installed without a mortice lock.
The mortice lock concealed in the door leaf is the basis for a door cylinder to be installed in a door. This is because the mortice lock contains the basic mechanisms for how a door works. Only with it can it be reliably locked, unlocked, opened and closed. Without a mortice lock, a door cannot fall into the lock because the mechanism would be missing.