I just don’t understand why people are making such a big deal about this card. Aren’t the vast majority of spells cast from your hand? Am I just clueless and don’t know how to play mtg? Someone please educate me.
One of the most popular formats for the game is called commander, where you build a 100 card deck with no duplicates.
One of the cards in that deck is a legendary creature called your commander that you will always have access to as it stays in a different zone called the command zone when not in use (rather than starting in your hand or deck), helping to make up for the reduction in consistency for playing a larger deck with no duplicates. As such, many commander decks are built to rely upon casting their commander.
Drannith magistrate doesn’t let your opponents cast their commanders from their command zone (which, again, is not their hand), which is a huge deal in commander. It is cheap enough to come down before players can cast their commanders (in most cases) if you get it early and it’s strong enough that there’s little reason not to put it in a white commander deck even if you aren’t playing a deck based on restricting your opponent’s actions.