I heard on a TV show dark is just for colour and light is just for flavour?
Light soy sauce: "is a thin (low viscosity), opaque, lighter brown soy sauce, brewed by first culturing steamed wheat and soybeans with Aspergillus, and then letting the mixture ferment in brine. It is the main soy sauce used for seasoning, since it is saltier, has less noticeable color, and also adds a distinct flavour."
Dark soy sauce: "a darker and slightly thicker soy sauce made from light soy sauce. This soy sauce is produced through prolonged aging and added caramel, and may contain added molasses to give it its distinctive appearance. This variety is mainly used during cooking, since its flavour develops during heating. It has a richer, slightly sweeter, and less salty flavour than light soy sauce. Dark soy sauce is partly used to add color and flavour to a dish after cooking, but, as stated above, is more often used during the cooking process, rather than after."
It's probably similar to the difference between run-of-the-mill balsamic and aged balsamic. In that case, the "light" or regular balsamic is an everyday condiment while the aged stuff is thicker and has much more concentrated flavor.