Why is microphone useful?

Answer 1

Original.

It changes the sound (pressure waves in the air in the room) into a digital signal.

Correction:

It changes the sound pressure waves into an ANALOG signal. Any digital signal is the result of feeding the analog mic signal into an analog-to-digital converter, typically in a computer's sound card.

Original.

An Electric Guitar doesn't need a microphone because it makes a digital signal.

Correction:A simple guitar with magnetic pickups creates an ANALOG signal from the motion of the strings, typically tens to low hundreds of millivolts. If you want a digital signal, you need:

1. Analog-to-digital processing in the guitar itself, or

2. Analog-to-digital processing externally, such as a computer sound card.

Original.

An mp3 recording is a digital signal.