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 that's right. We're using paper money to use in paying it even in investing because it is a money accepted by the government or what you say the only legitimate. So I think people continually use it because unlike bitcoin there are government that doesn't recognize bitcoin that's why a lot of us use paper money.