When you play the piano it is very important to position well your hands, fingers, wrist, arm and the whole rest of your body . A correct position allows playing better, with more facility and naturalness. Besides maintaining the hand and the arm relaxed you can play the piano for many hours.
Moving the fingers in a correct way and positioning the hand and the wrist in the best way guarantees to the pianist the possibility to play very difficult songs, to be wrong a lot less. Many piano songs are impossible to be played if you do not move in the correct way the hand and the fingers.
Hopefully that helped.