Some way to keep a count of the number of people in the building can be useful. I've seen occasions where someone checks to see if anyone else is still in the building, misses the one other person who is still there, and so arms the security system when they leave.
In case of an emergency -- fire, earthquake, active shooter, etc -- it helps to know if people are in the building. Having been in two of those situations and fire drills, it helps to know who was in the building and if they got out.
