How is the electric guitar useful?
It is easier to play due to lower action (distance of the sting from the fretboard). It is versatile and has volume and tone controls (usually). Its can have a tremolo system and it is easy to amplify. Plus they look cool!!!
If you mean "how is the electric guitar useful for life" then i can tell you that playing an instrument makes you brain sharper, gives you a sense of achievement and pride. It can open doors for you and will help you meet new people.
The difference between an electric guitar and an air guitar is a electric guitar is an actual guitar and an air guitar is imaginary. An air guitar is when people pretend to play a guitar with hand movements. An electric guitar converts vibrations of its steelcorded strings into electric current. These currents then go to a amplifier to make them louder.