Goku and Piccolo attempt to get their driver's licenses because Chi-Chi threatens to stop making them dinner. Both Goku and Piccolo panic at the threat and sign up for Driver's Ed. Piccolo is from Namek and it is made known throughout the show that Namekians only need water to survive. On King Kai's planet, when offered food by King Kai, Piccolo declined and continued to drink water. However, since Piccolo can consume Sensu Beans, he does have a digestive system, which means he can eat food. So whether or not he needs it to survive, he still may want it, and the threat still rings true.