Passengers who are checking in for flights are requested to empty their pockets prior to walking through the metal detector (magnetometer) to speed up the process of checking for compliance with flight safety and security rules.
The magnetometer is extremely sensitive to metal and will alert officers to any metal detected on a passengers person. Having passengers empty their pockets reduces the chances that each passenger will need to be checked with a hand-held metal detector and speeds up how fast passenger checks.
Metal that is detected after pockets have been emptied are commonly due to belts, watches, shoes or similar items. If the magnetometer detects metal after a passengers pockets have been emptied, checks with hand-held metal detectors should take less time.
The more efficient a passengers check in process through the magnometer, the quicker that passenger can proceed with their flight process and their travels.