Whether you work from home or in an office in a building with colleagues there are often important items that are personal or private that you'd like to ensure that no one else can get access to, or stumble across accidentally.
People often opt for a desk with lockable drawers where they can file away all of their important documents at the end of the day and then take the key with them when they leave, but you may feel that this isn't very secure if there is the possibility that others could be searching for something in your office.