That is what the wiper on the potentiometer is giving you.
A rheostat gives you a variable resistance from one point to the wiper, but a potentiometer's wiper provides the variable resistance in two different directions simultaneously.
From the Ground to the wiper will be one resistance, and from VCC to the wiper will be a different resistance (Unless you are perfectly centered)- Thus providing your voltage divider.
First Line 
A potentiometer is a three-terminal resistor with a sliding contact that forms an adjustable voltage divider.