Potentiometer is used as a voltage divider. If we connect Lead A to Vcc andLead B to ground then you can get voltages from 0 to Vcc by taking voltage at LeadW and LeadB. Mainly potentiometers are used to generate reference voltage for LM324. Suppose if you couple potentiometer to the shaft of a motor, then we can measure the angle moved by shaft by connect the output of Leads W and Lead B to an ADC to get a digital reading of angle. i.e a shaft encoder, but there is a limitation, we can't get rotation >270 degree and also number of rotations since potentiometer shaft can only move from A to B.