Central banks buy (government-backed) debt and issue currency (paper or electronic money/credits) in exchange. The government then spends these newly issued paper or electronic credits and because the inflationary effect of the these being introduced isn't yet felt the government gets full purchasing power.
If the population refuses to use this currency in favor of an alternative to that currency, then the inflactionary effect of the issuance of the additional debt occurs sooner and there is a loss of purchasing power that can be proportionate to the amount of funds issued. For example, if issuing $1T a year causes a $1T loss in purchasing power for the government's spending then there is no net gain from issuing the $1T in the first place. It only works if the inflationary impact of is absorbed by those users of that currency.
That's why countries impose currency and capital controls to try to force its population to use the government currency instead of investing in certain liquid assets or in sending their wealth abroad.