Ah, I see. Reasonable question. There’s a chapter (chapter 20) in the book “Fiat Ruins Everything” that addresses this. There they argue:
1) Speedbump/brakes on road to hyperinflation . If they just printed everything there’s more of a risk of hyperinflation whereas taxation slows this and keeps the system running longer 
2) Hide the Cantillion effect— they can hide behind calls for “taxing the rich” and try to conceal that that taxes and printing are just making those close to the money source disproportionately wealthier on net
3) Hide the inflation tax. Like a magician using smoke and mirror taxes hide the most fundamental/insidious tax of inflation