Yesterday in one of the two classes I teach, I had some volunteers come the front of the class and model the process I had just explained. Why is this worth doing, given that it's time-consuming and chaotic?
One reason is that it's just one more way of presenting information. Different ways work better for different people, and when a point is important I try to present it in as many ways as possible.
But there's a better reason for using students to model it. We're social animals. Almost from the day we're born, we find watching people to be more interesting than watching anything else. So, although many students will probably forget how I slid the model chromosomes around on the overhead projector, they'll remember the girls tied together at the front of the room, being tugged back and forth by boys with the yellow ropes and then released by the boy with the scissors. And maybe they'll remember that what happened to the girls is what happens to chromosomes.