At competitive levels you will know what is in your opponent's deck from the first few cards they play, or even class.
Stopping them from playing a key powerful spell can stop them from winning the game, especially if it is a combo piece. In MTG these exist and are sometimes played against decks which rely on one card to win them the game.
Being able to destroy a card in their hand is rediculously good and is currently played in every Black MTG deck, you can take that one card that is going to wreck your day - probally some kind of AoE or powerful creature. Usually there are conditions on what can be destroyed which make them less playable but not at the moment.
This is assuming you can choose the card to be destroyed.It can still be good if it is a random card from their hand as long as you draw more cards than them,it can also be good if it is a card from their library as long as you can get rid of enough to make them run out of cards.