Tapping is a core mechanic of Magic: The Gathering, and being able to tap your own creatures can be used as a resource, much like mana or your own life total
Again, it's worth emphasising the word "prominent" here
Plus, it has the little Atris, Oracle of Half-Truths minigame which I like very much
By cheating a Batterskull or Kaldra Compleat into play, Stoneforge Mystic gives more controlling decks a way to quickly end the game when they need it
Its a Commander variant inspired by the legend of King Arthur and Excalibur, where you pick a common creature to be your champion (commander) and a legendary Equipment to be its signature weapon
Most notably, Argentinas Diego Cassiraga won the Pokemon TCG World Championships in 2017 with a deck based around the Fairy-type Gardevoir-GX