The pot in this schematic is used as a variable resistor instead of a potentiometer. The resistance is 100% with the wiper in the "up-position" and 0% in the "down-position".
Well you don't need to connect 3 terminals to get a variable resistor, 2 is enough.
The point is, I think, that the voltage across it is limited should the track get dirty and the wiper not make proper contact.
However for setting a bias point this is not necessarily enough to prevent damage, you want to use the Vbe multiplier circuit with the pot arranged so that if the wiper goes open circuit the bias voltage drops, not rises. Even a fraction of a volt of rise in bias voltage can cook the output devices, a failed or dirty pot should lead to under-biasing which only causes reduction in sound quality, not thermal runaway.
The point is, I think, that the voltage across it is limited should the track get dirty and the wiper not make proper contact.
However for setting a bias point this is not necessarily enough to prevent damage, you want to use the Vbe multiplier circuit with the pot arranged so that if the wiper goes open circuit the bias voltage drops, not rises. Even a fraction of a volt of rise in bias voltage can cook the output devices, a failed or dirty pot should lead to under-biasing which only causes reduction in sound quality, not thermal runaway.