Its been a few years, but I once read an interesting book on how the credit system makes money on the vig, or float - the time between when they collect from the borrowers, and pay off the creditors, or vendors. This was in the 90s, before the Internet had basically reduced expectations and sped up payment systems. But ask your bank, anyone who can hold billions of dollars in funds for even a day can leverage that and collect a large amount of interest. Small businesses should know this too - I once worked for a man who insisted we pay our bills on the very last day possible, to keep his money in an account and earning interest as long as possible.