Revolving doors can be used as security devices to restrict entry to a single person at a time if the spacing between the doors is small enough. This is in contrast to a normal door which allows a second person to easily “tailgate” an authorized person.
Sometimes a revolving door is designed for one-way traffic. The door also revolves backwards to permit that person to exit, while also notifying security of the attempt.
Turnstile exit-only doors block people from illegally entering through the exit.