As a dogwalker, the privilege to let dogs off leash is very important to me. By letting them romp off-leash I am much better able to keep both the dogs and their owners happy by insuring that they get plenty of exercise.
Off leash recreation in city parks at off hours promotes the use of the parks at off hours - puts more people in the parks, and makes them safer. It is also humane and good socialization to provide dogs with this recreation.