I would also be delighted to have explained to me how borrowing money at interest is EVER in my best interests. Yes, we do 7 day or 15 day or 30 or even 90 day terms. But paying a late fee, or interest on it is acceptable only as a punishment for changing a contract after it is accepted. NOT a sensible part of the original contract.
You currently make $11/hour working as a cashier, which you hate. You got a job as a pizza delivery driver that pays $19/hour, which you would love, but you need a car to accept that job. You don't have enough money to buy a car, but you could borrow $2,220 at interest to buy a car.
You designed a cash-for-bitcoins terminal. You can buy the hardware for $1,900 and sell them for $2,400. You only have about $5,000 cash. You just signed a contract for 1,200 terminals, but you need to provide them all within three months. You don't have the $2.3 million you need to order 1,200 terminals, but if you borrowed it with interest, you could pocket $350,000 in profit.
Sometimes it is most efficient to produce value before you consume it. But it is also sometimes much more efficient to consume value prior to producing it, such that the amount ultimately produced is much greater. Interest consist of two people sharing this benefit because they both made it possible.