Uses of simple interest Recently I started to read again Brealey &amp; Myers's &quot;Principles of Corporate Finance&quot; (an older edition I hold it since I attended to the university course on Economics and Corporate Finance as a part of the curriculum in Electronics Enginering at the University of Bologna), since I need to evaluate the profitability performance of some projects I am involved with. The two authors use the NPV method which assumes (more or less explicitly) the use of compound interest, perhaps familiar to all of us, and, en passant, mention the existence of the simple interest. Inspired by curiosity, I searched for applications where the interest rate is required to be simple, but I was not able to find anything outside examples in textbooks and lecture notes, so my question is: what are the uses of simple interests in current economy?
Edit on behalf of the comments. I am asking this question because I am curious to see some examples and understand the reason why, in these cases, the simple interest is used. In my understanding, the use of compound interest is extremely natural since it corresponds to the intuitive concept that the invested money (continuously) produces a cash flow that is implicitly reinvested (at least up to the moment when the intial investment + the interest has to be payed back), thus it implicitly generates a (continuous time) conpound interest rate: for the simple interest I am not able to imagine such an intuitive explanation.
Interest payments are contractual payments. They need to be specified with an exact formula. For a given set of cash flows, one could come up with a multitude of formulae that end up giving the same values. In order to make interest rates comparable, laws, regulations, and market conventions specify that different conventions be used.
Of these conventions, simple and compound interest are possible choices. For simple interest, the interest payment is just the interest rate times the principle times a time factor. Compound interest takes into account increases in principal.
If a loan has associated interest payments that must be paid immediately, the loan resembles a generic bond: principal amount does not change. Bonds have a wide variety of conventions to convert the price of a bond to a yield, including a “simple interest rate” convention. However, changing the convention does not change the contractually defined price or cash flows. Since generic bonds do not roll interest payments into the principal, “compound interest” does not make sense.
For loans, interest is often rolled into the principal. We can then use simple interest or compound interest conventions.
Assume a monthly frequency, with no calendar date irregularities, for a loan agreement.

The interest rate could be specified as a simple interest rate of 1% a month.
The interest rate could be specified as an annual compound interest of $1.01^{12}-1$ or 12.68% (approx).

Regardless of the convention used, if the borrower makes no payments for a year, the principal will grow by (roughly) 12.68% since the lender will roll the interest into the balance at the end of the month, and then make the simple interest calculation based on the new balance. That is, lenders will still charge interest on interest in practice.
Which one to use is purely a cosmetic decision - is it easier to work with 1% a month, or 12.68% a year?