Indoor cats on average live longer than cats who go outside. Cats who go outside can be exposed to injury, illness or death by other outdoor animals.

Not only are they safe indoor from other animals, but also humans. There are people who aren't cat lovers and will try to poison or hurt cats who wander around the neighborhood.

Indoor cats are also safe from vehicles. 

Indoor cats are also less likely to contract other parasites and diseases from other animals outside.

Indoor cats are easier to keep track of when it comes to feeding because you know exactly the nutrition they are getting and don't have to worry about your neighbors feeding them too much.