Because if your turn your assets over 12 times (supermarkets probably do much better than this) per annum then your return on assets is 19.2%  - 1.6% x 12 (the risk -free rate on savings like fixed deposits is around 7% for a reasonable sum of money (9%? Forget it.).
If 50% of your business is funded by debt, then your return on equity (shareholder funds) becomes 38.4% per annum, which means you get yourt money back in 3 years - not bad at all.
Additionally, if your business is mostly cash / credit card (48 hour cash minus a small fee), but you pay your suppliers 60 days from invoice, then your customers and suppliers are funding a significant portion of your working capital.