There's definitely a large theater aspect but they also act as professional witnesses so to speak. They're trained to look for and record a suspect's characteristics that a lay person might not notice. A lay person might get you a description like "the big dude with long hair who got into a silver car" whereas a security guard will (should) get you a fairly precise height and weight as well as a plate number and the fact that the dude had a subtle tatoo on his wrist.

That's really reasonable. Thanks for the perspective!