Why do armored trucks have gun ports?

The series with Broderick Crawford ran 1955-1959. This episode was an armored car hijack attempt. The two guards outside were shot, leaving the third inside the back secure but pinned an a road that left him open to their repeated attempts to get at him. Yes, he had his gun ports to shoot at them, and they were crucial in slowing down the crooks enough until help arrived. Or so it seems the story went. I saw it a long time ago, but there you are.