The most usual reason to use the Facade pattern is for providing a single, simple entry point to a complex system. By introducing Facade, the client code can use a system by simply calling a single method/function. At the same time, the internal system does not lose any functionality. It just encapsulates it.

Not exposing the internal functionality of a system to the client code gives us an extra benefit; we can introduce changes to the system, but the client code remains unaware and unaffected by the changes. No modifications are required to the client code [Zlobin13, page 44].

Facade is also useful if you have more than one layer in your system. You can introduce one Facade entry point per layer, and let all layers communicate with each other through their Facades. That promotes loose coupling and keeps the layers as independent as possible [GOF95, page 209].