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.
I agree. A paper will not be valuable if guaranteed by the government as a legitimate currency paper that is permitted to circulate, and to make it authenticated that the paper is guaranteed by the government, they provide special signs that cannot be or are difficult to fake