Purpose of the Action of the Grand Piano
The purpose of the piano action as a whole is to accurately translate every nuance of the player's finger and hand movements into a musical note that reflects the exact intentions of the player. The slightest change in the speed of the stroke, the firmness of the stroke and even the release of the key must result in a corresponding change in the piano's performance.
The explanations here will certainly not make you into an expert piano technician but hopefully as a player you will have a better feel for the control of your instrument by grasping the sheer complexity of what happens everytime you press a key down and release it. A piano technician who can make the adjustments necessary to achieve the above described behavior from the piano is indeed a true artisan.