system
Given the following post, respond as if you're commenting on Reddit:
human
Why do we need central banks?

Sorry for my ignorance, I am new to monetary economics.
gpt
The theory is that central bankers and an independent bank have a mandate. This usually entails something along the lines of managing inflation.
Central bankers therefore set inflation expectations for the economy and their promises are more credible than if said power were under the control of politicians because they would likely use the power for their own gains. 
Theory is that this credible commitment to a given inflation target will trickle down to things like contracts between businesses within the economy and specifically wage bargaining contracts, thus reducing the inflation premium priced into contracts and inflation itself.

There is a lot of rather complex literature on central banks and monetary policy. This is just a very short answer.