What exactly do patrols do?

What exactly do patrols do? For better or worse, he was about to find out.

          – L.E. Modesitt Jr, Scholar

I guess patrols are patrolling, but what does that in practice mean? What does an engineer do? An economist? A manager? A nurse? A president? A vice president? A teacher? A barista? A beggar? A scientist? A programmer? A clown? An artist? A dancer? An actor? A singer? A taxi driver? A doctor? A psychiatrist? A soldier? A police officer? A train conductor?

I have an idea in my head of what each of these people are doing at work. It is probably just an idea though, and the thing I think they are doing is probably only a very small part of what their jobs actually entail. And I’m probably completely wrong on a few of them as well…