Total War is a helpful policy to adopt when playing as a domination-focused leader. Being at war with opponents prevents you from sending Trade Routes to their cities and gives them the opportunity to plunder any Trade Routes that pass by their units, but increased rewards for pillaging Districts and tile improvements and plundering your opponents' Trade Routes (which you'll have lots of opportunities to do as you send your military into foreign lands) will help keep your economy in order even when you're fighting wars on multiple fronts.
Note that since the April 2019 Update went live, you'll only receive additional Gold or Faith from pillaging and plundering unless you're playing as Harald Hardrada, whose leader ability allows him to gain Science and Culture in addition to the standard rewards for pillaging and coastal raids.
Civilopedia entry Edit
Ancient warfare was generally “total war,” until morality set in and rules evolved. Total war is defined as the mobilization of an entire civilization’s might and the application of that might against an entire enemy civilization – targeting not just its military, but the civilian populace and infrastructure, industrial and agricultural resources, and its will to carry on the fight. For centuries, professional armies and navies conducted “limited” warfare in colorful conflicts (and that’s not just those pretty uniforms), and chivalry and honor marked (supposedly) the battlefield. But with industrialization, the American Civil War and the even more entertaining First World War, utilizing every means to destroy one’s enemy became the preferred strategy. The Second World War was the quintessential total war (thus far).