Revolving doors are generally only found on the front of hotels and big buildings these days, so I'd always assumed they were there to help people with luggage or briefcases easily enter the building.
But they were created to be so much more than just a convenient way to enter a building- they're a clever way to regulate a building's air pressure and temperature, and thereby cut down on energy costs:
When revolving doors replace swing doors, they can save up to 30 percent in energy costs, or as Rockefeller University explains, “267 BTUs of heat energy, roughly equivalent to 1.3 hours of light from an incandescent lamp, 4.3 hours of light from a compact fluorescent bulb or .06 miles of fuel for an automobile.”
Creator of the revolving door Theophilus Van Kannel also saw them as a clever way to sidestep the issue of men opening doors for women, a social convention he greatly disliked.