Do you know the value of 100 baht today is not equal to value of 100 baht in next 3 years?
Why?
That is because to hold the money it also has "Cost". Cost of holding money is the "Opportunity cost"
How to find the opportunity cost?
We can use money to do other things to make more money such investment to do some businesses or may buy some shares in the stock market which these investment will return you the benefit in the future.
However if you decide to use your money to invest in those activities, it means that those activities should has higher return on investment than deposit money in bank. (To deposit money is the easiest way you can use to increase your money)
Therefore, If you decide to hold the money in your hand, the cost of holding will be the benefit you will gain when you deposit money in bank (Interest) so we can find the opportunity cost of money by calculate the interest of that money amount.
Today story will explain you more about this topic: