Why buy copies of Dark Banishing if you can go can purchase copies of Doom Blade or Murder instead
Samut takes full advantage of this, granting other creatures the player controls +X/+0, where X is their current speed, as well as making non-creature spells cost X less to cast, where X is their current speed
They are avoiding giving us the exact rarity, as they did with The One Ring, because that data point unlocks a whole lot of additional information for us: number of packs, how many copies of a card exist, things like that
The card will also be fine in casual Commander decks where its three mana cost isnt too restrictive, and could have further versatility in strategies that can reliably pay both costs for the card, like Jodah, the Unifier