Having a firearm as a police guard can make a huge difference in deterring crime and potential threats. Just having a deadly weapon as your sidearm will cause criminals to think twice about committing a crime at a designated location. It is also useful in the event that the guard had to use deadly force.
It's truly upsetting to me to think of living in a world where guns are a necessary part of protection, but it seems pointless to have a guard at all if he is stripped of any ability to defend against a person who is intent on violence. Even with criminals who aren't armed, if they have the intention to do harm to another, then they need to be stopped. A guard who is not armed has little or no means of authority to stop a criminal, and must resort to calling in someone who is armed.