It stands to reason then that a commander that can do both, and for a low cost of 3 mana, is likewise going to be quite good in many decks, and his color identity allows him to fit into a variety of strategies
If people were going to bring in a lot of overly narrow cards like Erase and Stain the Mind, he would transform into a creature deck to dodge these hate cards and take advantage of his opponent's watered-down deck
A lot of the new cards in this deck bring plenty of support for the monarch theme: Denethor, Stone Seer, Archivist of Gondor, omer, King of Rohan, Champions of Minas Tirith, Forth Eorlingas!, Crown of Gondor, Fealty to the Realm, and Oath of Eorl all bring some much needed support for the archetype, and in varied and interesting ways
While in Commander that might not be as high of a cost, in constructed formats like Modern and Legacy, it can be near impossible to do while staying in the game