Why

For example you can define Kilogram using Planck's constant.

But how do you define Dollar or Euro or Yen or whatever.

In older times we used gold, silver, copper coins but now we use a piece of paper that is covered with some ink.

I know the saying "Cash is king" and i know the benefits of cash.(Privacy and no transaction fees)

But i always feel we should use that piece of paper and ink for something useful.