It definitely helps, esp. for certain games like blackjack that uses the entire deck before shuffling. As articx said, it allows one to make an educated guess as to what the next card will be turned over. Since the number of cards is fixed as well as the value of each card, you can determine the probability of getting a particular card based on what had already been played, and thereby base your wager on this probability. That is also why casinos will use certain measures to ensure that card counting is more difficult... They tend to use more than a single deck of 52 cards so that when a card gets used, it doesn't affect the probabilities as much. They also use card-blanks to cut the cards (a piece of plastic with the same dimensions as a card) so that not only is the bottom card concealed as the cut is made, but also when the card-blank is dealt, the deck is shuffled thereby shuffling the deck before all of the cards are revealed. Some casinos also require a person to make a cut at least a third of the way through the deck so that at least a third of the deck will be unknown when the deck gets shuffled again.
Another way to "cheat" the casinos is to double your bet every time you lose a hand. By doing so, you will ensure that when you win, you will come out ahead. Because of this, casinos usually not only have a minimum bet at each table, but will also have a maximum bet. If you try to use this doubling rule, you usually will run up against the maximum bet within a few loses. And if you then move to a different table with a higher maximum bet, they'll be able to track your movements and eventually they may not allow you to play further or perhaps not to get more chips.