Reversed, or upside-down, cards are created by turning cards around as they're shuffled, giving you 156 possible cards possibilities instead of only 78. (Bear in mind there's no wrong or right way to use the Tarot, only personal preference.)
Why should I use reversed cards?
Because some believe reversed cards show where special attention is needed in a tarot reading, adding another layer of information.
Why should I use only upright cards?
Because others believe all the information you need for a satisfying reading is already contained within the 78 cards of the Tarot.