Purge is the Fallen Kingdom’s main way to sacrifice bodies. Once again this is not an economically efficient event, nor does it need to be. Be very aware of it being a move phase event, allowing you to do a lot of tricks with it, open paths and do all kinds of combos with your Undead Archers.

Don’t be afraid to use Purge for just a single sacrifice, especially if it's a Cultist as 3 damage for 2 cards is not the worst deal, particularly if that helps you get a jump start to your Undead Warriors by boosting them or allows you some great plays with things like Sacrificial Pyre or Undead Archers.

Most of the explosive turns of Fallen Kingdom involve a Purge, hopefully while Sacrificial Pyre is active also. If you are not playing at least one Purge every game, something is not right, and if they show up in your hand at good moments you should be using two Purges per game.
Often the biggest problem with Purge is losing the board position of the unit(s) you sacrifice, so hopefully you have a couple of Undead Archers ready to take the positions of the purged units.