Center vents have a military heritage. When a man traveled on horseback, his coat was designed for comfort in the saddle and a center vent in the back permitted the tails to fall on either side of the horse.
Center or side vents also make access easier to the trouser pockets
As the least expensive vent to manufacture, center vents are the most common vent in off the rack clothing
Side vents are more expensive to make and are generally seen in custom clothing. Side vents also give the illusion of greater height.