Why can't I use just the digital pins and a ground to use a potentiometer?
A potentiometer is an analog device, meaning it communicates its position by varying the voltage rather than sending discrete ones and zeros. 

If a linear potentiometer is turned half way then the output of the pot will be at half the voltage of the input. If it is turned all the way then the output will equal the input voltage and if it is not turned at all then the output will be 0V. 

In order to make sense of the potentiometer you must connect the output to the analog pins and it will give you a 10 bit number, the higher this number the closer the output voltage of the potentiometer is to 5v (or whatever you set the Analog Reference voltage to be, but don't worry about the AREF pin for now).

Digital pins are either on or off, there is no in between, so they do not work for this application

This page has an excellent potentiometer user guide: https://www.arduino.cc/en/Tutorial/Potentiometer