Have you ever noticed that when you put your earbuds in backwards, not only is it the wrong shape in your ear, but the sound is completely different for some songs  for the left and right ears? Headphones are made to handle not only mono signals, which result in both ears hearing the exacts same sound, but stereo signals also. This results in the emphasis of the bass and the performer(s)' instruments in the left ear and the singers in the right. This produces a fuller sound than mono signals which is often the intention of the music industry because headphones and earbuds are very limited in the sound they can reproduce. Headphones help create the illusion of a bubble around the user, and the use of stereo signals help create a more in depth experience for users.