I've run such a program for certification.
Basically, the manufacturer is trying to determine if there are any 'human factors' issues that might not have been detected during the design and internal testing process - things that would be likely to show up when external pilots (with varying degrees of familiarity with the type) begin to fly the aircraft.
The data collected can sometimes result in minor changes to the flight compartment design or nomenclature, the training programs, and the documentation for the aircraft. It is also very useful when the time comes to develop a MMEL and/or CDL for the type.
Michael