Perhaps we should think about monetary policy this way.
If all prices were perfectly flexible, monetary policy wouldn't matter much. Monetary policy matters because not all prices are perfectly flexible, which means that bad monetary policy causes monetary disequilibrium, which is what happens when prices want to change but don't change. Recessions and booms are examples of monetary disequilibrium.
What we want is a monetary policy that minimises monetary disequilibrium.
But monetary disequilibrium is a theoretical construct. We don't observe it directly. So we need an empirical proxy for monetary disequilibrium, so we can tell the central bank to minimise that empirical proxy.