Case 1: It's easy to use mouse for normal people, but it may be hard or even impossile for the disabled ones, especially the one loseing both hands.
Case 2: Some boys/girls just wanna let their hands free(maybe eating, writing or doing something else and have no hands to control mouse), they can use the Head Mouse
Head Mouse is just like a earphone, it can be put on one ear and control the mouse. High accuracy gyroscope is inside, it will sense head motion and convert to mouse movement. Muscle activation sensor (EMG sensor) will get the mouth muscle signal, it will convert to mouse click. Bluetooth will help transfer all the informations.
So, move your head, mouse pointer will move; grit your mouth, will finish single/double click.