At first glance this card may seem to work against you, however there are a lot of cards that work well with "Dark Designator", such as "Confiscation".
Use this card and then use "D.D. Designator", "Card Destruction", "Morphing Jar", "Confiscation", "Mind Crush" or another card which forces your opponent to discard. Declare a monster for this card which you know your opponent has in their Deck and has a lot of use for (either because you can guess from the cards you've already seen, or because you sided this card in after the first Duel).
Use this card while "Magic Hand" is in play to send the searched monster to the Graveyard and inflict 800 points of damage.
You can use this card to declare the name of a card you know your opponent won't have in their Deck. If it's not present, then you can check your opponent's Deck to verify. And read their strategy along the action
This can be used to weaken an "Exodia" Deck. Use this card to put "Exodia's" head in your opponent's hand. Then use either "Exchange" or "Amazoness Chain Master" to bring "Exodia" to your hand. Since all "Exodia" pieces are limited to one in the Any Format, and you keep this card in your hand, it is now impossible for your opponent to win. Most "Exodia" Decks forgo cards that require a discard, as well as hand destruction cards, meaning it will be impossible for them to retrieve the stolen piece. However, use this with care, as there is the possibility that your opponent has the other 4 pieces and is simply waiting for the last piece, which can be disastrous if you bring it to their hand.
Or you can declare a card that would not be in an "Exodia" deck to check your opponent's Deck and track how many pieces they have at any one time.