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
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 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
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
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.