What is purpose of maintaining common SD distribution channels and common divisions?

Common Distribution Channel and Common Divison are maintained so that if any master data like customer or material maintained with respect to one distribution channel can be used in other DCh. It prevents the multiplication of master records.
Eg: 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…the same for materials also.