Land, natural resources, militaries, taxation systems, GDPs, etc.
A better example would be what's gold linked to? Previously, it had worth due to its uses within industry & products, but these days there's substitutes for most applications that previously used gold (ie. computer chips / boards). Nowadays, it only has value because people believe it does, and people are willing to accept it in exchange for barter / currency.
Same goes for bitcoin. Or hell, beanie babies, or pet rocks, or exotic fish, or artwork, or whatever...