Anyone who has a house cat knows that they shed all year round. You can vacuum and vacuum all you want, but that cat just keeps shedding in winter, spring and fall. You're right if you thought that a cat's fur was supposed to insulate it from the cold. The fur not only insulates them, but also provides a barrier against physical and chemical damage to the skin, and protects them from harmful sunlight.
Shedding is actually influenced by everything from genetics to nutrition to environmental temperature. But research has shown that the main factor that determines when a cat sheds is the amount of sunlight outside. Changes in sunlight cause the cat's brain to signal its hair follicles to respond appropriately. When there's less sunlight, cats start growing short, fluffy secondary hairs whose job is to provide insulation. When there's more sunlight, cats start shedding.
Indoor cats are exposed to less natural sunlight and more artificial light; their bodies lose track of seasonal changes, and they shed constantly year round. Good nutrition, lots of exercise, and plenty of grooming can help keep your cat healthy and happy. However, shedding is a natural part of the hair life cycle.