Shuffling cards is a tricky business. It's also a lucrative one for gambling casinos.
In a game such as blackjack, an astute player can try to memorize the cards already played to have a better chance of predicting which cards will come up later, thus potentially gaining an advantage over the dealer and the casino. If the cards aren't properly shuffled and their distribution isn't truly random, the advantage could be even greater.
Indeed, to keep games moving along at a brisk pace, blackjack dealers don't always take the time to perform the seven riffle shuffles necessary to achieve an adequate level of mixing (see Disorder in the Deck: Disorder in the Deck). This means that certain sets of cards may remain close enough together to be tracked through the deck, and players can use such vestiges of pattern to their profit.
To counter card tracking and dealer miscues, casinos introduced the use of multiple decks and mechanical card shufflers