A face wash is different from a moisturiser. As far as I know Face Washes clean all the dust from the face. It is very essential fo people who are in the outdoors and in the sun. The sweat and the dust combine and your face starts looking black and dark.
It is to remove this dust that Face Wash is used.

The best way to use a face wash is this-
Wash your face with cool clear and clean water and wipe it clean.Then apply the face wash.A good quality face wash will wash away the dirt from the face leaving your skin fresh.