In addition to the more technical answers you got it also serves to incentivize consumer spending or investing. If you put your money in a bank you get interest, or you invest in stocks and increase the value of your investment over time.

If you cash your paycheck and throw it in a box for 20 years the real value of your money (how many goods you can buy) will have decreased. Therefore the peak value of that money is usually right when you get it. It serves to keep money going back into the economy. Silly example: you have 5 dollars. A can of beans is $1. You can get 5 cans of beans. In 5 years due to inflation that can of beans is now $1.25. Now that $5 is only worth 4 cans of beans.

The rich look down on the paycheck-to-paycheck consumers but really they kick a lot of money right back into the economy when it's at peak value.