I use these during investigations, but they could be used during any centre, station or rotations session.
The idea is so that students have a visual for how many students are allowed to be working at any given time.
The black dots are used both as a visual aid for number but I have also used them for management. Each student is given a peg with their name on it. When they go to a new centre they clip their peg onto one of the dots. If they are all full they then have to wait for someone to leave.