It makes the owner / rider think they are doing something great for the horse.
I fail to see any good in it unless there is heat or inflammation or a recent injury of some kind that needs to be 'cooled out'. Then, 'go for it' and apply DMSO after cold hosing or icing. Otherwise, put the horse up, let him drink and eat and rest and go home an do the same.
If there is not some obvious injury, I like to leave things alone and check for heat the next morning very early.