Slippin' and a slidin'
Mechanical potentiometers, whether serving as front panel controls or as trimpots on circuit boards, can develop intermittent slider connections. If the slider connection becomes an open circuit, whatever that slider is connected to could exhibit unpredictable behavior.
Figure 1 An intermittent slider will lead to an indeterminate output.
It would be prudent if in anticipation of such a part failure, that one include a shunt resistance from the slider connection to a safe and predictable voltage, in all probability, to ground.
Figure 2 Adding a shunt resistance keeps the output predictable in case of slider failure.
Suppose that the potentiometer's resistance was 10K. The added shunt resistance from the slider to ground might be chosen as 1 Meg. That shunt resistance would be large enough for the resistor to not materially affect the potentiomter's normal function but small enough to pull the next device's input connection to a safe and controlled level.