Why do earphones have designated left and right ear?

Answer 1

The left-ear speaker's made for your left ear(and vice-versa) so you can hear better.

Stereo music is mixed with the intention that certain instruments will appear in different locations in the the stereo spread (somewhere between left and right). To accurately recreate the intentions of the music producer, you must put the headphones on for the intended ears. Orchestral music is recorded in stereo with a specific arrangement of instruments on the stage (violins on the left, for example). To recreate the sound as you would hear it from the audience, you must put the earphones on the correct ears.

Answer 2

To match the original stereo recording, the way it was produced. Otherwise, the sounds would be reversed. In some cases it might matter as to the fit of the earphones as well.

Answer 3

Because it's how you hear the music. It doesn't really matter which side you put in your ear, it just sounds better when you do it properly. =D