Page "Potentiometer" Paragraph 46

One of the advantages of the potential divider compared to a variable resistor in series with the source is that, while variable resistors have a maximum resistance where some current will always flow, dividers are able to vary the output voltage from maximum () to ground ( zero volts ) as the wiper moves from one end of the potentiometer to the other.

There is, however, always a small amount of contact resistance.