Most physical prototypes have some sort of user-facing interface, typically on the top of the device. If your prototype only has one knob or button to control the inner workings, a text label explaining its function might not be needed. But what if you have three knobs? What if there are 20 user-facing control elements, knobs, buttons, and sliders? In that case, you absolutely need labels to explain what control element does what.
Personally, you might find it relatively easy to remember what the individual controls are for, but your friends and family will have a hard time controlling your prototype.
There is always the possibility of using pen and paper to explain the different control elements, but writing ...
