The holes in the pic allow forager bees to continue to enter the hive, without losing new foragers.
If you use a robber screen as a moving screen, you need to close it late in the day or early morning, or you lose the field force. You can put a moving screen on at any time of the day, wait a little, then move the hive.
Thanks specialkayme! I had looked at that picture and wondered about it. I can see what you're talking about. The inside bees will focus on the most outward area of the screen where there are no holes whereas the incoming bees will move to the area that is sloped inward closer to the entrance and find the entry holes. Neat.