I know this isn't too important or anything, but it's a modding dynamics thing:
The ability to have variants of a creature without messing with the number of a type of creature. Common example of variant creature: dragons. In most games, there's dragons of different 'colors' that denotes abilities and difficulties. Currently to get 5 color dragons I need 5 entries in the raws. That leaves, when chosing megabeasts, dragons in general at a 5/9 chance of being picked. But I LIKE the other megabeasts. To get the ratio back up, I can change frequency values, or add more entries of the other megabeasts. But megabeasts don't have frequency values (in fact, sasquatch has frequency of 1, so megabeasts would need fractions anyway if they worked under that system.) Now there's all sorts of megabeasts. Are they now, overall, more likely to be chosen in worldgen?
The remedy: in the raws, the ability to have variants. Instead of 3 entries of dragon, it could have:
Creature:Dragon
ect.
Variant1: Black dragon
name:black dragon:black dragons: black dragonian
ect.
Variant frequency 10
Variant2: Mauve dragon
ect.
Variant frequency 5
And when a world is generated, it will only look at this =after placement=... Thus it will place based on only a single creature:dragon entry, and only after things are placed, apply variants, and then play out the history, as it does, with this particular individual dragon.
While I use megabeasts, an extreme example, this applies to many things, from the ability to have 'a large buck' vs 'a mid-sized doe', or a stone golem vs a metal golem vs a mud golem without having three entries in the raws, or to more quickly and easily add variants like platonic elemental variants to magical creatures.