The passionate action-oriented monks of the Jeskai, the Whispers using Temur magic, Nagas in the Sultai as protectors (and avengers) of the jungle, Mardu military prowess including supporting roles (and that dragons can come in many shapes and sizes!), and Abzan clerics regrowing the lost kin trees
This is it
Those collecting Pokmon TCG will be wildly familiar with the concept, but it's where retailers will take liberties with the "recommended" part of an MSRP, and list it for what they think folk will pay
Some of the standout cards even rival those found in Secret Lair drops, giving players unique options and added value
Reactivity doesnt necessarily mean preparation: you can frame it as opportunistic, in the way that more villains should just be shot while monologuing
Firion, Wild Rose Warrior fills a similar role in an unorthodox way