Players create a token that's a copy of the creature under control, before sacrificing it at the beginning of the next step
This common card now goes for a fair bit of money thanks to its prolific use in the old Channel Fireball decks, back when the game had just 40 card decks and no real restrictions on other very powerful cards (looking at you Black Lotus)
This means you cant use the mana right away without shenanigans, and it just slows this down too much
This can be beneficial if all your creatures fly, but it's also helpful if opponents attack each other, letting you profit from the action
I have put Yawgmoth, Thran Physician on 7th place on my personal list, but I can see that he deserves better
While Kiora, the Rising Tide sees play in Esper Goryos Vengeance, Emet-Selch can potentially replace her as a stronger alternative