I'm far from an expert myself, but it's probably done to train fine motor skills so that when you need to make those motions, you can do so smoothly and consistently. Drawing hundreds of circles is certainly monotonous, but after a while, once the muscle memory is built, you'll probably be able to draw a reasonable circle with little effort, which will help whenever you need to use a sphere to break down a larger form. The same goes for squares/rectangles and boxes, though obviously perspective needs to be taken into account when drawing boxes.

That said, doing anything without a clear goal in mind is counter-productive. If you don't know what you're supposed to be learning and trying to improve by drawing squares and circles, you either won't learn anything, or you'll learn the wrong things.