Nothing more than it's widely accepted and that it's backed by yet another widely accepted from of currency (usually; gold). Some systems don't back it up, IIRC.
Because it's widely accepted, there's a degree of stability built into it. People generally agree on its value.
This is all gross-oversimplification, I realize, and I don't particularly want to get into the weeds on this topic, because it's only of passing interest to me. Regardless, my points around 'real' money being more stable and accepted are valid.