@burcidi-- Both koikuchi shoyu (dark soy sauce) and usukuchi shoyu (light soy sauce) are all-purpose soy sauces. So you can use them in cooking or as condiment sauces.
However, if you use soy sauce in cooking to lighten the flavors of seafood or meat in the dish, it is better to use koikuchi shoyu than usukuchi shoyu. Since usukuchi shoyu is less sophisticated and less fruity in taste, it is not as good in masking the heavy flavor of meat and seafood in dishes.
Usukuchi shoyu is not just light in color, but it is also light in flavor. This is because the salt inside doesn't allow it to ferment as much as koikuchi shoyu. Just like an alcoholic that is not as fermented, soy sauce that is not as fermented is lighter and less sophisticated in flavor.
I recommend usukuchi shoyu for people who don't really enjoy the flavor of soy sauce in their food, but they have to use it as the recipe calls for it. It's also good if you don't want the food to be colored dark by koikuchi shoyu.