By giving an unlimited outdoor access, you are letting your cat living its own life.
Cats are natural hunters and they can spend up to six hours a day hunting, chasing or just stalking. And of course, after spending that amount of time outdoors, your cat will never grow fat. Have you ever seen and obese outdoor cat? :)
You also don't need to clean after your cat.
They can have the most fulfilling lifestyles by being outdoor, however there are risks.
Your cat can be hit by a moving vehicle which it can end up being dead. If not a car, injuries can come from people who hates animal or from being chased by dogs. Cat fighting for territories is another danger.
Even the cat's own curiosity can be dangerous. Cats can be exposed to poison by eating poisoned prey or foods that have been exposed to poisons i.e. garden chemicals.
Disease and Parasite
Usually from random mating :) or from other stray cats.
Lastly, you might loose your cat. If you have a beautiful Siamese Cat, it is enough reason for others to 'adopt' it.
Tags: injury, poison, parasite, hunting, stalking, , cat