Sacred Haven of Debauchery

Is this when the enemy army has a lot of fig creatures, and you're running around with only vampires, for example?

This is one of the options. Another example is to "bench" a green dragon and bring out a red one, because the opponent has many fire-breathing creatures. Or, all your units are level 4-5, except for the Incas. And then you face an "invincible" horde of enemy units at levels 1-3. It's more advantageous to "bench" the Incas and bring out, for example, the Gloomwings. Then you can simply scare all the opponents. :)