The short answer is so your listening experience more closely matches live sound (or action if you are using your earphones for watching a movie).  When you go to a live music event, your listening experience will be affected by where you are sitting in relation to the music source.  If you are sitting closer to violins, they will likely stand out to you more so than whatever instruments are farthest from you.  When music is recorded, the goal is to replicate a live music experience, so sound engineers will mix the various sounds to one channel or the other (Left or Right) so your ears hear different parts and you get closer to a "live" listening experience.  

Does it really matter?  For music, audiophiles will likely answer yes and I'm sure they will give you a very good reason why.  To a lot of people, however, there might not be as much importance.  Here's where it matters and I have a great tip for you to make it easy to tell the right ear piece from the left ear piece.  If you were standing by the road and someone approached you from the right, your right ear would hear them approach and you would likely react in that direction. Same goes in a movie thanks to sound engineers who try to replicate real life so you have a better viewing experience. If someone enters on the right side of the screen you will react to the right side of the screen because that's where the sound comes from when your earphones are in the correct ears.  I watch movies on my laptop when I'm flying so I always pay attention to which earphone is in which ear.  When I work out or use my earphones for running, I wear ear hook earphones that are only designed for one side or the other.  I'm in my fifties now so I don't see as well as I used to and the tiny L and R are not easy for me to see anymore.  My solution is to use two different color ear tips so I don't have to squint to know the black one goes on the left side and the clear one goes on the right.  Actually I recently started wearing our memory foam ear tips so black is still left, but I use a gray one on the right.  I thought I had come up with a great idea and then I came to know Pro.2, one of the earphone companies we now sell here on our website.  They put different color ear tips on so they are already set that way out of the box.  And here I thought I came up with some brilliant idea.  Oh well, maybe I'm not so brilliant, but I did get the most beautiful woman in the world to marry me, so I've got that going for me!  Try the two colors to identify right from left; you'll very quickly realize it's so much easier than squinting.  Thanks for stopping by earphonesplus.com today!