That's because paper currency are believed to have value and are also legitimate by the government, so paper currency can be used to buy all your needs. Not because of the paper, but because of the approval from the government and also a belief that it has value.
yes right, not because of what money is made but what makes money valuable is the trust of all parties to an object that is considered to have value to be used as a medium of exchange and buy something you need, no matter the money is made of paper or metal, or even virtual like crypto currencies, as long as there is still a lot of trust in exchange rates that can be called money