Headphones are important sometimes.  They allow me to listen to an audio source without anyone nearby hearing.
Yes, I agree. They are especially useful when travelling on a bus or airplane.
They convert electrical signal to a corresponding sound.
Have you ever tried the version that vibrates your eardrum to create sound? I tried Google Glass and that is how it worked.
No, I haven't.  That's interesting I didn't know that's how google glass worked.  It's an interesting hands free format.