Besides, there were plenty of occasions where untapping a Mountain for his +1 was all the mana you needed, enabling a Galvanic Blast to keep Koth safe and well on his way to ultimating
In addition to potentially allowing you to cast an opponent's strongest spells for free, Tasha's static ability creates 3/3 Demon tokens whenever you cast a spell you don't own, converting these stolen spells into additional manpower
It can also be useful in Commander to clear a full stack since you can choose different targets for each copy, potentially letting you stop multiple players
But you still get a 1/1 vampire token with lifelink, so you get small blockers against creatures without trample
Teferis Protection, which will protect not only your whole board, but also your life total from changing
rather than have the game automatically do that for you