Why would you short two pins of a potentiometer?
Tying the wiper and one end pole together effectively turns the potentiometer into a simple variable resistor (or "rheostat").

It no longer functions as a voltage divider would, but it does mean that in the case of a failure of the wiper contact (intermittent contact with the carbon track), you'll still have a known maximum resistance value, instead of becoming an open circuit. 



Depending on how your cooker works this might be a crude failsafe to reduce the heat in case of a failure of the wiper.  I would still expect other failsafes to be in place too of course.

It is not unusual for a 2 pole variable resistor of equivalent ratings to be more expensive than a potentiometer, so to save cost sometimes a designer will choose to use a pot and simply tie the wiper to one end.



EDIT: Thanks to @winny for reminding me that "rheostat" is another name for a variable resistor.