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A valet key is primarily designed to limit access to certain areas of a vehicle while still allowing functionality for a valet attendant or service person. This is particularly useful in scenarios where the owner wants to maintain some control over the vehicle's use, such as restricting access to the trunk or to the glove compartment where valuables might be stored. By using a valet key, the owner can ensure that the person parking the car can operate the vehicle without having complete access to its interior, thereby enhancing security and convenience.

The other options do not accurately capture the primary function of the valet key. For instance, while a valet key might operate basic functions, it does not fully operate all vehicle locks. Additionally, a valet key is not solely intended to start the engine; it has broader functions related to vehicle access. Lastly, while it may serve a backup role, its main utility focuses on conditional access rather than purely as an emergency solution.