During world war i, buying war bonds was a way for people to

 War bonds were the way the U.S. financed much of the war effort. There were war bonds with pages of squares for 25 cent war bond stamps. The booklet had room for $25. These were deposited in the bank, and following the war they could be cashed-in with interest. By this method the government was borrowing money from its citizens.
Basically to make sure that their nation would have enough money to fund its war, to lead to victory.