The imposition of taxes, which must be paid in government currency, is precisely what gives government currency its worth.

Then why does it have worth OUTSIDE of the country as well?

From a Chartalists p.o.v the State creates money, spends it via appropriations and then taxes the community creating a demand for the currency.

While that is workable, it implies that the sole or primary purpose of taxation is to create value for the currency, which is absurd.