The government's fiat currency would be worthless without a complicated system of taxation in order to create demand for it. No private individual or business would be interested in selling their goods or labour to state and local governments in exchange for fiat, unless they are forced to have that fiat in order to pay taxes - and here is included mortgages to the banks, since it is 100% a tax as well.
This is what people forget about (especially tech minded people), when they think up new systems of taxation that would be more fair and effective: Any fair and effective tax would destroy the currency, since people and businesses could easily pay their share and then instantly convert their remaining wealth into non-fiat assets, or even safer fiat assets. In order to prop up the value of the fiat currency, the tax system needs to be extremely bureaucratic and reach into every and any kind of economic activity imaginable. That way there is a constant need for businesses to keep that fiat on hand and do their accounting with that fiat.