Covering the deck with the hand allows the holder to manipulate the cards so as to view the top card's face (useful in poker or blackjack) or to lift and select a card without the spectator's knowledge (useful in card tricks).
Using a hand to cover the deck allows for viewing the top card of the deck, which is useful for games like poker or blackjack. It also enables the holder to secretly pick a card from the deck without the spectator knowing.