In the February 7, 2018 Update, Raid gained the effect of doubling yields from pillaging Districts when Sack was removed from the game. Prior to that, it only doubled yields from pillaging tile improvements.
This Military Policy is specifically designed for aggressive militaristic civilizations. Pillaging tile improvements is a great way to hinder the enemy, while at the same time making some lump sums of Faith, Gold and other yields. The Raid Policy doubles these yields! What's more, once you conquer the enemy territory, it is pretty easy to bring the pillaged improvements back online (a single Builder suffices, since its Repair action doesn't spend Charges). This is not the case when you pillage districts, for example.
To pillage, plunder, and generally demoralize a foe - there’s nothing like a raid. Since the barbarians couldn’t stand up (usually) to open battle with the armies of the Greeks, Persians, Romans, Chinese, and other civilized sorts, their incursions tended to take the form of extended raids. In time, these and other empires also used raids against the barbarians, often dispatching auxiliaries to devastate their lands … driving off their livestock, burning their camps, and worse. So well did this strategy work that some “wars” were nothing more than a series of bloody raids, and the practice became part of the military repertoire that survives even today.