Some strategies in Yugioh rely on 60 card decks. They aren't competitively viable in the current format, and they are normally very mill-heavy (sending cards from your deck to the graveyard). These decks tend to have a lot of effects they can activate in the GY and essentially use it as a second hand. "That Grass Looks Greener" specifically allows those types of decks to dump up to 20 cards directly into the GY at the beginning of the game, which is normally game determinative.
Sometimes other rogue or pet decks want to run 60 cards as well, due to the bulk of their engine being too large to cut down, format specific strategies and the large number of staples you need in a deck just to have a chance to play the game.
Some strategies in Yugioh rely on 60 card decks. They aren't competitively viable in the current format, and they are normally very mill-heavy (sending cards from your deck to the graveyard). These decks tend to have a lot of effects they can activate in the GY and essentially use it as a second hand. "That Grass Looks Greener" specifically allows those types of decks to dump up to 20 cards directly into the GY at the beginning of the game, which is normally game determinative.
Sometimes other rogue or pet decks want to run 60 cards as well, due to the bulk of their engine being too large to cut down, format specific strategies and the large number of staples you need in a deck just to have a chance to play the game.