In my case, it allows my horses to continue to live outside with run in shelter and turnout rugs, but still allows me to ride moderately in the fall and winter without them sweating through their coats and overheating.
There are several different versions, the one you posted is what I would call a very high trace clip. My horses have blanket clips, which leaves more hair than what you posted.
It allows to be able to ride and have your horse cool off faster while still being able to keep most of your horses coat and have them still go outside with a blanket and not get too cold.
I had to trim my horse very similarly to that this past winter cause she would sweat when I would just walk her. She grew a very heavy coat and live in Southern California right now. Where during the day it would get up in the 60s but at night it would drop to the 40s. This way with a light blanket on at night she could be warm enough but during the day when it warmed up she would be able to keep cooler.
As well as the above reasons already mentioned, it also means that they dry off quicker than they would with a full coat, and it stops them getting a chill. I believe it's more common here in England, almost all of the 22 horses we have at work are clipped in some form, apart from the youngsters and the 30 year old!
Jdw, horses in the wild with full coats don't do forced exercise that causes them to sweat through their coats.
So if you want your horse to keep his full winter coat all winter, that means you can't ride him past a walk under saddle, at least in my part of the world. Not an attractive option for many of us.
******ing the coat growth with blanketing is a partial solution, but I still had overheated horses after moderate work, so a blanket clip - clipping the hair from the head, neck, belly and flank area, but leaving what's normally covered by a blanket, is a better solution.
Clipping the horse completely, triple blanketing and stalling 12 - 16 hours a day is another solution, just not the one I would choose for my pleasure horses.
I'm wondering how the poor horse regulates his temperature when he's clipped. Part of his body is ok and the clipped part is cold. It's ok for a horse to sweat into a heavy coat but most owners don't want the extra work of helping the horse dry out. A wool cooler wicks moisture away and walking helps the horse dry from the skin outward as does some intermittant munching on hay. A horse has the abilty to raise and lower his hair, plus turn it to catch cooling breezes or fluff it up to hold warm air. Clipping takes this all away.
I am curious as to what your riding discipline is, where you're located, your actual experience cooling out horses with a full winter coat after they've sweated through and your actual experience with partially clipped horses.
'cause I've got a trunk full of wool coolers, and with my horses in my climate, all the wool coolers and more walking than riding doesn't get the job done.
Saddlebag - try going for a run in a thick woollen jumper. No problem if the horse isn't a performance horse, only raising a sweat under saddle once in a while, but a horse that is worked 6 days per week including heavy work is going to get uncomfortable with a thick, full coat, which can lead to health concerns.
Also keep in mind, not everyone has time to stand around waiting for the horse to dry before rugging it after a ride. I know I certainly can't stand around for an hour after a ride, I've got work to do! Another thing that should be mentioned is that a horse taking a long time to dry, will get cold very quickly while still wet. But a clipped horse will dry very quickly and can have rugs on before it becomes cold.
As for the ability of the coat to keep the horse warm - that's why we use rugs ;)