Indoor cats do need some vaccines, even against Rabies! Their indoor status does mean they will be less likely to be exposed to some diseases, but that is not the same as protecting them.
Let's talk about Rabies first, because Rabies is a uniformly fatal disease that has no treatment and is transmitted to people by their pets. Rabies virus is transmitted generally by saliva to blood contact, most commonly by a bite wound. The virus enters the body and travels up nerves to infect the brain, effectively poking holes in the brain of the patient and causing death by encephalitis over 4-10 days. Rabies can be expressed in the dumb form- blankness, drooling, lack of expression or the mad form- drooling and blindly attacking/biting. The incubation time (from bite to sickness) can be days to months after the bite.