Their presence itself is a valuable deterrent, and they are also fully allowed to do consent searches and customer stops and interrogations as long as they don't block an exit. They can also act as a valuable witness in a criminal case against any shoplifters if required. And that's just for store employees working as security guards; more and more big-ticket stores are using actual police officers which, obviously, have the same power as any other police officer.
No reason to roll your eyes, it's all pretty simple, logical stuff. No store security employee undergoes anywhere near the same amount of training as an actual police officer, so they obviously don't legally deserve anywhere near the same amount of responsibility or power.