Simple interest is useful for planning parts of your financial future because if somebody owes you money, each day they don't give money back to you, you can add, for example, $1 to the amount of money they owe you. This will be important because it can help you maintain a well organized system where if somebody is not able to pay you back right away, they can give you the money at another time but you they will have to pay interest so you will get more money than they owed you at first.
