If you do, the card gets sacrificed, and you get to take an extra turn
P/T Number 14 Language English Legal Formats Commander, Legacy, Vintage Not Legal Formats Alchemy, Brawl, Competitive Brawl, Historic, Modern, Pauper, Pioneer, Standard, Timeless Rulings (11/10/2023) Indominus Rex's first ability will give it only one of each kind of counter, even if you discard more than one creature card with the relevant ability
This factor has seen the Hitmonlee card be ever-present in fighting-type decks, which hit knock-out-worthy damage for the majority of players in the game
Given that you typically play it when you don't have other plays in aggro, and as such, it is a lot better than doing nothing
This is worse when phasing out exactly one creature since it costs 2 mana, but it can be much more impactful, plus you can burn cards in your hand for extra targets