¿Por Que? Because not using one can lead to "open grounds," which can cause spurious voltages to sometimes creep into the pins your testing for voltage values

¿Por Que? Same issue as pull-up resistors

Reference

Interestingly, the poteniometer, since it's hooked to ground, will act as a "voltage divider," (which it would not do if you didn't hook up the ground). This gives reading between 0 and 1023 with the Arduino no matter what Ohm value you use for the poteniometer.