In this case, sacrificing a creature nets you two black mana, which is a useful ability in a wide variety of decks, and it represents the color black perfectly
Plus, if things get hairy, we can drop The Unagi of Kyoshi Island, a massive 5/5 Flash threat with Ward Waterbend (4), making it a nightmare for opponents to remove without tapping down their own blockers
For many, the Sun and Moon Era GX cards are among the best of their kind
But trust me, the only ones who are going to make money off the McDonald's cards are the early birds, the ones who rush to market with a full collection or suspiciously sourced box of stock before people realize what a bad deal this is
For instance, you might use a creatures tap ability to deal damage or draw cards right before it would be destroyed by an opponents spell