From the publisher's website:
You can never have too many monster options. It’s a truism of running a campaign. Not only do you want to keep your players guessing and keep creating new challenges for them, but you also very likely want a broad range of thematic choices from which to draw. It’s not enough to have just a few CR 5 choices. Instead, these gamers want a few CR choices for every type, theme, and role a monster might play. After all, if you are running a campaign built around the rise of the goblinoids, you need lots of different goblins to populate your adventures. The more monsters to choose from, the better!
But sometimes you have the right monster for an adventure, and it has the exact look, the perfect theme, and the tailored feel, but you can’t use it because the CR is wrong. Even if a manticore is exactly the guardian you wanted for the Lore Vault of the City of Refuge, if you’re running a CR 9 adventure, a CR 5 manticore won’t work. You do have options, of course. You could run an encounter with 4 manticores, but too much of a good thing can become monotonous and make a fight drag. You can advance the 6 HD manticore to CR 10 by taking on about 8 HD, but that’s a complicated process that involves adding new ability score increases, feats, skill points, and possibly a size increase. If you plan for characters to face the same manticore (or its kin) several times, the effort might be worth it, but not for a single encounter.
Or, you can just tack on the “mighty” simple template, which takes less than a minute.
Simple templates are designed to require a minimum of rewriting of a creature’s stat block. In essence they are all designed to work like the quick rules of the monster advancement templates presented in the Bestiary. In a few cases this might lead to creature statistics not matching exactly what they would be if the same monster was built from scratch. For example, the sample missing creature, the missing lynx, is a magical beast, but its statistics are still largely those of a creature of the animal type. Although this is an inconsistency in numbers, it has no effect on how the creature plays within the game, and the extra work required to convert d8 Hit Dice to d10s isn’t the best use of a GM’s time. Although the type of a creature can have a real impact on play (interacting with a ranger’s favored enemies, bane weapons, and similar game rules), and its hit point total is important, the size of its Hit Dice doesn’t actually matter.
Presented within this product are six simple templates that can give GMs options to boost the CR of the monsters they need, create new threats their players aren’t expecting, and multiply the usefulness of the hundreds of monsters already at their fingertips. Having a two-headed template doesn’t replace the ettin, but it does allow for two-headed dragons, twin-skulled canines, and even freakish multicranial golems. The simplicity of the templates keeps them from being a chore to add, and they allow a GM to create new foes almost on the fly.