I had been on a lot of invigilation duties lately. What is invigilation for?

I understand that it is important to have invigilators around when that is an official examinations going on. We are there to ensure that the examination proceed smoothly. We are also there to answer any queries that the candidates may have pertaining to the examination. But someone commented that what we really have to do is to stare down the students and make sure that they get the message that cheating is not an option.

Now, am I supposed to be an invigilator or a sentry?

I feel that we need to have trust in these students. But are they trustworthy? I like to believe that they are. But I don’t have any means to verify it.