In a lecture from Paul McGreevy he told us that collection was used in the days of the old war horses to help them balance better. Due to the heavy weight of armour on the horses front end and on the rider it was necessary to move the centre of gravity backwards and place more weight on the hind quarters than on the front legs. Also an arched, open back is more capable of carrying a heavy weight than a closed concave back which is why bridges arch up rather than down. So for a war horse carrying a heavy weight for a long time, whilst not initially natural, collection should be a more comfortable way of travelling. Thus, as with most other things in dressage, it originated from a war time role for horses. I agree that today it is less required except for a few noticeable exceptions of rider physique at my yard
As to the comfort/discomfort proposal I would relate the story of my own injury. I broke my hip badly in a fall last year, and took 6 months to recover. During my physio it was made very clear to me that I had to walk with a good gait, even when on crutches and not allowed to bear weight on the bad leg. It would have been much more comfortable to limp or hop along but I endured the discomfort in the knowledge that in the long term it would result in greater comfort of movement. I now walk without a limp and have just come back from a winter climbing holiday with my godson, so it worked.
Horses, however, just want to feel better now and don't care about the future. Thus if a horse moves badly, but it takes greater effort to make progress towards moving better with a view to easy times ahead they would rather remain as they are. It is up to us to have the knowledge, as my physio did, that temporary discomfort now will lead to greater comfort in the future.
This isn't only for physical issues, but mental issues as well. For example a horse that gets really stressed living in the human world by being asked by humans to move their body, like Filly, will remain stressed about it the rest of their lives, unless we can teach them that it is ok to move when a human asks. Then the mental stress will be lessened. What she knows at the moment is that if she reacts badly then 9 humans out of 10 will back off and she gets instant comfort. Wouldn't it actually be less stressful for her overall if she could just learn that being asked to move is fine and nothing to be defensive or worried about ? But to get there it will be necessary to invoke the stress she is reacting to and temporarily make her less comfortable. Depending on the hill of discomfort that has to be climbed this can either be a fairly mild process, or pretty extreme.
The most extreme example I saw was James Roberts taming a stallion. This was a very very difficult horse, dangerous in the extreme. I saw it pick a vet up by the leg after she had sedated it (sedation aggression it is called) for dental work to be carried out! The "conversation" James had with this horse took it to a place of extreme discomfort before he could convince it that being lead gently was a much nicer place to be than rearing, striking and fighting all the time. All this horse was doing is what it had learned to do in the past to get back to a comfortable place as fast as possible. It had no idea that there was actually a much more comfortable place of "being" until James took him there and showed him, but that was on the far side of the discomfort mountain. As James said afterwards "do you have the emotional control to go there with the horse and immediately relax back to neutral when required?".
He strongly suggested to the owner that it was gelded as soon as possible to prevent injury. To my knowledge they did not take this advice
and now James is gone I don't think this horse will have a nice or particularly long live.
I don't offer this idea up as a mandate to go out and make horse lives uncomfortable for no purpose, that would be abusive. What I am suggesting is that with this knowledge and idea it has made me more emotionally able to cope with pushing Filly, a horse I waited 2 1/2 years to own and cried like a baby when she was temporarily taken from me, to places she would rather not go. It is hard for humans (me anyway) to put stress on a being they love, but this idea has allowed me to put stress on Filly knowing that in the long term she will actually feel better for it and have a better live. It is what allowed me to train her to actually go out in a field again and be with other horses. She had been stable bound for 8 months before I started helping her. It took six months to achieve and she would have much rather stayed in her stable, her only place of safety and comfort, during much of that time. But we got there and she loves her field time now.
When this is done there has to be a clear purpose to it and a real knowledge of what the final product should look like, without those it is just abuse, however mild.