Just because your cat doesn’t venture outdoors doesn’t mean he/she will not come in contact with a virus. Certain viruses are airbourne and we see indoor-only cats contract these, presumably through screens or human transmission. While your cat may not be at high risk for Rabies virus, even indoor-only cats are required by law to have this vaccination. Finally, cats are notorious for slipping outside when you least expect it. Without current protection, they are then at risk for disease transmission.
Ask your veterinary hospital to discuss the benefits of vaccination with you.