Think about it from first principles. What is money? Money is nothing more than a proxy for goods and services. So, how much money do you need in an economy? Well if money is a proxy for goods and services, it is no good for the supply of money to stay fixed while the amount of goods and services expands. Indeed, the money supply has to not only expand with the present volume of goods and services, but also with peoples' willingness to enter into contracts to trade goods and services over some future time frame (i.e. loans).

So the money supply must generally keep growing in a growing economy. How should that be effectuated? For a long time, we tied expansion of the money supply into how quickly we could mine gold and put it into vaults. Given the exponential growth of the economy, and the distinctly non-exponential nature of gold production, it's obvious that wasn't going to work for long. So we have what we have now, with a central bank controlling the money supply.

That's the justification for the existing system. Are there alternative mechanisms that don't involve someone with their hands on the big money supply dial? Maybe, but I haven't seen any convincing ones yet.