Imagine you are an sized e-commerce merchant. Someone checks out on your website and sends you a bank transfer. Then what?
You go through your ledger (bank account), find the transaction with an order ID as a memo (hopefully the buyer added it and correctly), and then you look up that order in your system and submit the order form to your warehouse?
It works like: customer gets an order number and puts this on the description of the bank transfer. This is parsed by the accounting SW coupled to this bank account, and voila!
Most banks charge some amount per transaction on commercial accounts though.
The problem is that it's extremely failure prone and not instant.
Gotta wait for the buyer to make the transfer, you gotta wait for the transfer to go through, you gotta hope that the buyer put in the order number in the description.
You go through your ledger (bank account), find the transaction with an order ID as a memo (hopefully the buyer added it and correctly), and then you look up that order in your system and submit the order form to your warehouse?