First of all you need to be sure that your horse has Cushing disease. This is an hormonal problem that is very commun for old horses (above 15 yo).
There are a lot of symptoms such as: long hairs that don't fall, a lot of sweat, drinking a lot, not eating but having a big belly, loosing muscles and urinating a lot.
The horse needs to be treated because he suffers a lot (not directly from the disease but what goes with it like founder and a lot of infections because the horse is more fragile). If he's not treated, he will suffer a lot, have a lot of problems that will make you see the vet very often and he will be very tired, eventually it will cause his death.
The treatment helps him having a better life quality, it is a daily lifetime treatment that also requires a balanced nutrition (a lot of fibers and not much carbohydrate).
The diagnosis is in favor of the horse if the disease is treated soon enough, unfortunately, it is very complicated for the vets to detect it because the disease progresses slowly.
I hope i could answer your question, don't hesitate to ask your vet for more details :)