You can pick a bad card in your opponent's Graveyard to put on top of your their Deck, so they will have a dead draw.
This can work wonders together with the other "Watt" monsters, who have several hand destruction effects. By depleting the opponent's hand and putting a bad card in the top of their Deck, they may have a completely dead hand.
You can also pick a good card out of your opponent's Graveyard to put on the bottom of their Deck, so they cannot get it back so easily.
Another option is to pick a card whose effect activates in the Graveyard so that your opponent can't activate it.