There are many reasons and you mention a few. Monty always long lines the horse after its first Join-Up and before his first rider up.
This is an excerpt from From My Hands to Yours:
the horse. At this point, you are prepared to drive your horse on the two lines (Goal Number 8). Swing the offside line over his hips, which places you well behind the horse and in the driving position. You should be experienced with long lining long before you execute this procedure with a raw young horse. I recommend that you long line many trained and quiet horses before you take on a young inexperienced one. If your horse accepts the lines well, you are now prepared to allow him to circle the round pen while you stand near the center guiding him. Your horse should successfully execute circles on each lead and turn comfortably away from you as you request.
When your horse is turning and accepting his work at the walk, trot and canter, you should stop your student and turn him outward facing the fence. You should allow the animal to stand for a moment and relax. Then, lifting the lines, you should encourage him to take one step backward as he draws the slack from the reins. I recommend that you always cause your horse to take a step back before going to the next goal in the training procedure. I feel that this is important, as it instills in the horse’s mind a desire to relax at the conclusion of each maneuver. If you accomplish this with your horse, it is likely that you will never produce a horse that barges into the next procedure unilaterally; he is more apt to wait for you. I recommend driving, using the rings of the Dually halter once or twice (see Chapter 7), and then advance to the rings of the snaffle while long lining.

Hi
I'm Kelly Marks, the Monty Instructor from the UK. I recently bought a horse that couldn't get in the showring because of his nappiness (jibbing) and rearing. One of my (excellent) students re-started him starting with Join Up (not easy at first as he wanted to bite all the time!) and about 10 days long lining. We've carried on working him for 6 weeks and he just came 4th out of 20 in his first dressage test. I don't know how to post photos here but I think this gives you a link to one http://ihdg.proboards.com/index.cgi?board=ght10&action=display&thread=104745&page=1

Of course, all the concepts/techniques came into place but in this case I think long lining was key as you could make the changes while being in a safe place!

I always thought it was just for exercise!!!

In dressage, double lungeing and long lining are great ways to teach more advanced moves without the weight of the rider. A more sensitive hand-mouth contact can be developed; and bend, balance, and collection improved. Long lining also can be used to start a driving horse.

Long lining's great to get horse used to other things like going through water or over tarps, scary things - driving them down and out of the arena for the first time, they seem to stay calmer on their own in the lines than with a rider on their back and there's less chance of them being booted too hard for not going forward or having other confusing messages given by the rider.

Lunch hour for me is when I study,.....
I have been watching the long lining lessons and I am totally fascinated. Apart from the energy Monty displays himself and the joy with which he does it, a lot of things are beginning to make sense.
Thankyou for sharing your dedication, Monty!
Christy