Depending on the situation, Harpie Dancer can be used to swap out material for an Extra Deck summon, return a vulnerable Monster to the hand, or get back a Monster with an effect that activates in the hand
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If, for any reason, an Aura token that would be created with Arna Kennerd's last ability can't be attached to the appropriate creature, that token isn't created
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
As such, knowing the basics the matchup is useful