Paper money is useless in the same way gold is useless. If you have wads of paper money in your pocket, or purses of gold, it will not buy you anything, if people are not willing to accept it as payment for their hard work or ingenuity.
Try not accepting paper money as payment for your hard work or ingenuity. Try demanding gold or bags of potatoes as payment, and you can be sued in many countries. Paper money is legally enforced as payment for all debts.
If it weren't for that subtle piece of compulsion people would boycott paper money very soon. Because the only thing paper money facilitates is to help governments wage wars, meddle with export rates, pay their friends in the industry etc.