Nothing to do with side reins and a halter. Every time you work with a horse you have an opportunity to help the horse learn things.. from how to use his body to proper movement on a circle. Lunging can be a great opportunity to help a horse learn so MUCH.. transistions, tracking up, shifting his weight to the hind quarters, collection, transitions and so forth. A halter is simply not able to give you any sort of clear communication with the animal. It is, IMO, pointless for this training.
BTW I learned this all as part of what I call Horse Training 101 stuff back in the dinosaur age... from a person who was an accomplished Horsemaster from England.
There is something about the outside of a horse that is good for the inside of a man. ~Winston Churchill
) Dinosaur Horse Trainer