Here's why:

* Theremins produce a very weak electrical signal: The theremin's internal circuitry generates an oscillating signal, but it's not strong enough to be heard directly.

* Amplifiers boost the signal: An amplifier takes the weak signal from the theremin and makes it much louder, allowing you to hear the sound clearly.

* Amplifiers add tone shaping: Some amplifiers specifically designed for theremins offer features like equalization and effects, which allow you to shape the sound of the instrument.

Think of it this way: The theremin is like a microphone. It picks up your hand movements and translates them into a signal, but you need an amplifier to make that signal audible.