The theory behind it is that if the student can be comfortable, they can focus on the problem more than if they were uncomfortable. Many people like this because you can take responsibility for how they learn, if it’s on a yoga ball, standing, or just plain sitting in a chair. But it can increase productivity and encourage the students to try out different ways to sit. I have been in a classroom with choice seating, and it seems to work well, as the people in standing desks get to move a little bit and stretch out more. People on stools work just as well the students in chairs. It can also allow the students to be less robotic in the sense that they all have to deal with things the same way, in the same, possibly uncomfortable seating format.
For all of the teachers who have their students all sitting in the same type of chairs, and have them all feeling sluggish as the day goes by, you might want to experiment with the choice seating concept. It gives your students the ability to choose the way they learn, comfortably.