The open slats in the video certainly provide good exterior visibility. Why would an intruder want or need to open the window in the circumstances or would you suggest for audibility - hearing a getaway vehicle pull up or some sort of warning sound?
With me not being very mechanically minded, do you how a person opens only a few slats on the aluminium shutter as you can't do that with a Venetian blind?
The problem is that we are not able to carry out a few experiments in the actual apartment so it is necessary to speculate a little.
It could be auditory, for example to hear someone approaching the front door (if necessary an intruder could exit via the sliding door), or just a general listening check to see if there is anyone about. Another possibility is to get closer to the shutters so it is easier to see through the holes.
Fiona Payne talks about adjusting the shutters in her rogatory:
"And I think we only ever sort of slightly opened, it was one of these shutters where, erm, sort of graded, erm, you can open it a little bit and it just opens up with a few holes to let a little bit of light in but the whole shutter is still actually down. And that's all we ever, we never opened the shutter, we just, we'd open it a bit in the morning to let a bit of light in and then shut it, erm, you know in, in the night-time to the point where it would only have a very minimum bit of light coming in"