This is important in places like the US where you are not allowed to drive with headphones (both) but its ok if you have one in ear as part of your hands free set. If they were both the same length then one would dangle and drag on the cord while driving. Having uneven lengths allows you to have the other across the neck dangling while not affecting (and actually relieving weight for) the shorter one while it is in the ear. It's nice.

Because of the weight relief it sits better than the old single earpiece headsets of my old Nokias.