The lunging work, when done well, develops new neural circuits.
Together with in hand work the horse learns how to use its body in a different way. New questions will be asked to the horse, and new answers will be found by the horse through a kinesthetic approach.
Lightness is the final end result as well as a correct spine rotation!
When done correctly, lunging and the work in hand are an excellent tool for restarting older horses that have become difficult under saddle.