One of the most common problems riders face when turning is losing control of the horse's outside shoulder. This exercise is a great way to get more control over that outside shoulder to help make more accurate turns - which will help you come out of the turns straight and in better balance.
A good halt is square because it was a good halt. Not because the rider has taught it the "trick" of squaring up after an unbalanced halt.
If your horse finds lateral exercises more difficult in one direction vs the other (as many do), then they are not truly straight, or evenly laterally supple.
"I used to ride mostly Thoroughbreds. I don’t know if they’re more intelligent, but I find that they think for themselves a bit more than the warmbloods. With a warmblood I find I definitely have to do a lot more, and think a lot more for them.
In the sport of Eventing, ensuring that our horses are truly fit for our level of competition is one of the best ways to prevent unnecessary injuries.