Common Distribution Channel and Common Division are maintained so that if any master data like customer or material maintained with respect to one distribution channel can be used in other Distribution Channel. It prevents the multiplication of master records.
Example: A customer is created for say sales area 1000/20/00 then the same customer can be used in sales area 1000/30/00 if we maintain 20 as common distribution channel. Hence no need for extending the customers. Similarly no need for extending materials.