This can be extremely valuable when you have a lot of Treasures but you need to find better spells to play, or when you know what is the top card of your library and you need to get rid of it because it's a dead card
I see this working best in Mono-Blue Terror lists, since the deck can have a tough time getting rid of things on a permanent basis
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Grim Flayer: That's one powerful bear
You can see an overview here: 90% (4th card), 60% (5th card) 5% (4th card), 20% (5th card) 1.666% (4th card), 6.664% (5th card) 2.572% (4th card), 10.288% (5th card) 0.5% (4th card), 2% (5th card) 0.222% (4th card), 0.888% (5th card) 0.04% (4th card), 0.16% (5th card) So, if you open 1,000 boosters, there should be one to two cards of the crown rarity