It's a symmetrical effect that stops players being able to cast more than one spell on each turn
Its admittedly not the greatest rate as it only kills small creatures, but the debuff acts as a combat trick that discourages larger creatures from getting in, plus picking off that Orcish Bowmasters might be essential
Oathsworn Knight is also another potential three-drop for more aggressive styles of Mono-Black Devotion it has two black mana symbols and the fact that you have Gray Merchants in your deck means they cant just take the damage forever
It's quite mathy, but he goes into a great deal of detail and explains his rationale, while not ignoring the realities of the game (there are one-mana Cycling cards, cards that fetch lands, cards that tap to add mana which aren't lands, and more.)
A booster pack contains 12 cards, with two being rare or higher, plus a foil card in the last slot that might be an Enchanted