The steel-man argument is that the monetary policy of central banks can cause hyper-inflation of a fiat currency, and holding onto an asset that is immune to that, and potentially even being able to transact with that asset is a reliable way to break free from the monetary policy of the central bank. The fact that one may place this transferable asset into a centralized institution that we may call a (lower case b) "bank" isn't at odds with the aforementioned principle.
Put another way, the dollar doesn't depreciate because of my credit union, it depreciates because of the Fed.