The way it works is simple: if your opponent has a Pokmon on their bench that has taken damage, you can swap it to the active slot
Finally, because he can make many spells and abilities free, he can be the third card in numerous infinite combos, like Carrion Feeder and Gravecrawler, Necrotic Ooze with Asmodeus, the Archfiend in your graveyard, or even just K'rrik and Blood Celebrant, which essentially makes a free mana of any one color for each three life you pay
Unlike typical abilities or keywords, Monarch is a designation a player can hold, transforming the flow of the game by introducing a high-reward, high-risk element to the multiplayer format
To be fair this does work in the TCG to some extent, just compare the current price for a Secret Illustration Rare Iono to the price one commanded at release date (or if you want to be completely insane, during prereleases)