A cost centre allows you to break a customer down further by stipulating a location, office, department etc.
For example, your customer has requested their calls be separated between their London and Brighton offices. By creating a cost centre for the London office and a cost centre for the Brighton office, you can then assign the customer's CLIs to the respective cost centre in the Caller Identification section.
When your customer receives their invoice, they will see a breakdown of costs between the two cost centres.