I think we've all questioned it, right? Well it turns out there's a legitimate reason why headphones have left and right ear tips - and it's not to just to do with comfort.
In stereo recordings, it is said that sound coming from the left sound louder in the left channel. Although audiophiles might disagree - it's not particularly important when it comes to music but it is essential in TV; watching a car drive across the screen would be mildly confusing if the sound was reverse in your ears, right?. Another reason might be that music wants to accurately deliver music to you as you would hear it in a studio; some instruments i.e. a guitar may be located slightly more to the right on stage/in the studio. Really, it's all about making sure you have the best listening experience possible!