When turning a card face-up for its disguise cost, its classed as a special action
You can certainly play Commanders with Polymorph/Scramble elements that aren't dedicated to the strategy, such as with Neera, Wild Mage or other similar 'hit a big thing' decks, but the power behind playing a dedicated, limited choice for what you intend to hit off of a flip can greatly boost a deck's capabilities, and win games out of nowhere
And it contains a solid collection of offensive, defensive, and utility creatures for almost any situation
The Commander: Captain America, First Avenger Captain America, First Avenger has a very cool triggered ability that lets you attach any equipment on it at the beginning of combat, meaning that expensive ones like Argentum Armor can be equipped with no effort
Artificial scarcity also plays a part