When you reinsert a bill into a vending machine, it is typically done to complete a transaction. If the vending machine only accepts bills and not coins, reinserting the bill ensures that the machine registers the payment correctly. Additionally, if the machine spits the bill back out the first time, reinserting it gives the machine another opportunity to accept the bill and process the payment successfully. By reinserting the bill, you are following the standard procedure for using the vending machine and ensuring that your transaction goes through as intended.