A meeting room booking app is the mobile client of a room booking system. It shows which rooms are free right now, books one in a few taps, and syncs the result back to the calendar so there is only ever one record of the meeting.
It sits alongside two other booking surfaces rather than replacing them: the calendar, where most recurring meetings are set up, and the room display panel at the door, which handles walk-ups and shows the room state to anyone passing. The app covers the case the other two handle badly, which is someone away from their desk who needs a room in the next ten minutes. Check-in and auto-release from the phone are what stop those quick bookings turning into ghost meetings.