Lets assume you’re toasting white bread and you want it a little bit toasted - a long way from burnt. Which way do you turn the dial? Towards the black toast picture where the indicator is thickest? Or towards the white toast picture where the indicator is smallest?
It’s not what you’d hoped and you’ve just burned 6 slices of bread.
With a rheostat.
Turning it clockwise increases it’s resistance which decreases the current flowing through the heating element, in turn decreasing the heat generated by the element; vice-versa for rotating it counterclockwise.