Why does the sensor have a third leg?

Question

Arduino

Perhaps a stupid question, but ... Please explain why radio components need a third leg (for example, a piezo tweeter. I can control the sound without it) ;{

Answer 1

Judging by the diagram, the module is a voltage divider, where in one arm there is a photoresistor, and in the other (between the "signal" contact and the "ground" contact) there is an ordinary constant resistor.

Answer 2

Power, ground, signal.