A turnstile, also called a baffle gate, is a form of gate which allows one person to pass at a time. It can also be made so as to enforce one-way traffic of people. Turnstiles are used at a wide variety of settings, including stadiums, amusement parks, office lobbies, airports, and casinos.
From a business/revenue standpoint, turnstiles give an accurate, verifiable count of attendance.
From a security standpoint, they lead patrons to enter single-file, so security personnel have a clear view of each patron. This enables security to efficiently isolate potential trouble or to confiscate any prohibited materials.