An assembly might be made of several different materials and some of them might not be able to chemically withstand a given plating bath.. 

Different finishes can be specified on a given part.

It is often more economical to plate a precious metal only where it is needed rather than over an entire part. The center of an IC lead frame is one example.

To avoid excessive buildup on fine machine threads.

To block blind holes.

Achyutha has pioneered in masking and plating with a acurate dimensions and within the provided tolerances.