This blanks a lot of powerful cards in many different decks, and although it isnt as popular as it may have once been, its hard to do better than free to cast and free to activate
Sometimes you don't want a card to solve itself, and other times Occam's razor proves true, and you just want more life-gain-matters effects in your deck
Mystical Tutor puts cards on top at instant speed which means you can essentially cast any noncreature spell in your deck for once you successfully attack with Narset
I can even see this as a powerful card in an Esper artifact shell, with a bunch of mana rocks and Voltaic Key effects dropping it quickly and then you running cheap artifact creatures to sacrifice
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While a blue player would need to hold back mana in early turns to use Compulsion, there are no such restrictions on the Looter