I'm one of those people who do not agree with cats being outside unsupervised. Cats can and do get injured in the great outdoors. They can get into cat fights or fights with other animals. There are many dangers facing domesticated animals in the "wild". In a city, many cats are poisoned or hit by cars.
Another reason is the risk that cats pose for our wildlife such as birds. Cats are natural predators and do prey on birds. In some areas, bird populations have been affected by cats.
Our cat is allowed to go into our backyard as long as one of us is out there with her and is supervising her.