Ah, the age-old question of "if you can print money in any quantity, why even levy taxes at all?"

The answer is, that would completely destroy the value of the fiat money, people would see it as the garbage that it is and stop using it altogether. But when noone uses a money, then printing it brings no economic benefit.

So they need to keep the inflation low enough that people tolerate it, and for that, they provide at least some of the financing through taxation.