Central billing is a phrase used to describe the process of using the collective buying power of independent businesses to extract discounts from suppliers.
[1] A convenience store retailer takes delivery every day of bread, milk and other perishable items.
In return for this the supplier makes a payment to the wholesaler to reflect these benefits, called a rebate or Long Term Agreement.
Instead of having to monitor many daily payments they simply have to write one cheque a month for potentially thousands of transactions.
Secondly they offer a valuable service to retailers to whom they also sell other items to through their traditional wholesale channels.