There are multiple reasons to put a collar on your cat. If your cat gets lost or injured and is wearing a collar with an ID tag, it will be easy for someone to find you, and your cat won't get mistaken for a stray. Even if your cat is microchipped, don't discount the fact that some people might not think to take a cat they found to the vet to check for a microchip. You can also use collars to hold bells, GPS tracking devices, and tags that unlock cat flaps (so that strange cats can't come in). Another reason to put a collar on your cat is to make him more visible in low light. Choose a reflective or light-up collar for this purpose.