Passenger tracking is very valuable for airports for the purposes of analyzing traffic patterns, density and dwell times. These parameters help the airport plan schedule maintenance, monitor and control crowds, and help design future expansion plans for terminals and common areas.
Passenger tracking is also useful to measure amount of passengers in checkpoint lanes to notify pasengers of wait times and manage the number of people working at the checkpoints.
I have managed the deployment of these systems by using W-Fi as well as cameras.