yes, It is vital because vaccines which are used for cats make enough immunity against very dangerous diseases includes: Panleukopenia (FPV), Herpes virus infection( FHV), Calicivirus infection (FCV) and Rabies.
FPV, FHV and FCV are very contagious diseases which lead to a disaster for your cat.Rabies is a killer disease which is transmitted by biting an another animal which is sick like a cat , a dog or animals which transmit the virus such as bats, skunks and raccoons.
Even an indoor cat is in danger because viruses of these diseases can be alive in environment for long time which can be transmitted not only by other cats but also by other animals to your house.
Don’t forget, prevention by vaccination is better than treatment at all.Unfortunately the writer visited some indoor cats which suffered and died from one of these diseases because their owners were thinking vaccination was not necessary for indoor cats.
There are some other less contagious viral infection like Feline Leukemia Virus and Feline Immunodeficiency Virus which can be prevented by vaccination. These diseases can be transmitted to your cat if it contacts with infected cats in long term.Therefore vaccination against them can be omitted for an indoor cat.