Machines are variable. They'll beep for shoes one day, not another. Hence you'll be asked at certain times to take off your belts and shoes (and even once something with a lot of zip). I've even seen a conveyor belt full of chunky rolex's because the staff wanted the heavy watches off. It's about speeding the pax through.
Machines will also randomly beep for a check. Which is annoying for me as I'm a really quick security check-thru guy. I never set off the alarms me!
Unfortunately, some machines will inform the security staff that it is a random check (AMS for example). This could be bad from a security point of view as you don't want the security staff to know why they are searching someone, only that they should be. It's to be expectations. There's another school of thought on the subject but its not one I agree with that says the security staff should expect to find something on every search otherwise they lose faith in their machines. DISCUSS....