Question on the Federal Reserve System in the United States

[KonstantinosM]

How does changing from a fixed supply of money to a variable supply of money help?

I understand that before the Federal Reserve System 1 dollar would be worth the same year after year, decade after decade.

Burying 100 dollars and taking them out of the ground 100 years later would give you the same spending power.

How is it a better system to make the currency vary according to supply and demand when the currency itself does not represent a financial good?