Employees who deal with the public, like cashiers, receptionists, security guards, hotel staff, etc. often call male customers "Sir". It's supposed to show respect. For example:
Sir, can I get you something to drink while you wait?
In the U.S., you can also call any adult man who you don't know "Sir".
Excuse me, sir. I think you dropped this.
The equivalent title for women is "Ma'am".