You are correct, setting the I and D constants to zero makes it a P controller rather than a PID controller.
To answer your question, sometimes a P loop is all that is needed to get the system behaving the way you need it(although usually at least the D constant is also required in FRC robots). It's important to note that when you are tuning a PID controller, you typically start with the P value and leave the I and D at 0. For more information about tuning a PID controller, I'd recommend
this Wikipedia article as a starting point. It's not the only way to tune a PID controller by any means, but it is the most widely used.