Deck patches are usually 2/3rds the length of the board from tail to nose and provide more strength for the deck where your feet and knees pound on the deck. They are not necessary but sometimes added for heavier footed surfers. It of course adds more weight to the board but a lot more strength.
Normally a deck patch is the first layer of glass to go on the foam and cut in a triangular or rounded shape towards the nose (2/3rds from the tail). Leaving the deck patch edge running strait across from rail to rail will cause a break point in the board. One or 2 more layers of glass are then rolled out over it and laminated at the same time.