To get objective data about users' attention and physical state.
Using specialist hardware such as eye-tracking equipment, eye-tracking glasses or sensors to measure heartrate, skin conductivity or brain activity, can be used to collect objective measurements of users' engagement with a design.
Eye-tracking is used in addition to standard usability testing to detect problems in the interface. These can be a starting point for interviews. Other biometric measures are used to measure player enjoyment in games or in simulators.