The copper plating is a stopper for case hardening, ie. the carbon will not diffuse through the copper plating.
It's used on low carbon steel components where only certain features require hardening.
Typically the component is rough machined followed by copper plating. Then the feature to be hardened is machined such as cam surfaces and gear teeth as you mentioned.
Next step is case hardening, a process which diffuses carbon into the surface of the steel, followed by quenching.
The depth of the case may be from .003' TO .030" deep depending on type of steel, carbonizing process, and length of time in the carbonizing atmosphere.
The real beauty of this process is that gears can be made from a steel which, when heat treated normally, would provide an exceedingly tough steel, but lacking the hardness required for wear resistance. By case hardening the gear teeth only one gets the best of two worlds, extremely hard wear surfaces combined with a tough and impact resistant core structure.
Automotive gears are typically made this way.
Cheap cutting tools and dies can be made from hot rolled steel followed by case hardening; but it won't have the tough core.