Peggy Sue is the real expert here but Ill take a stab at why we do it.
The feeds we are mostly drawn to these days...the popular brands like safechoice and strategy are supposed to be a complete feed. They've got a bunch of different ingredients, all which are supposed to be beneficial to our horses. Here is what I have learned from the great people on this forum...(like peggy sue)
Most of our horses really don't need all the 'grain' we give them. I learned this lesson the 100% hard way. I had a gelding who was extremely hot and he had every right to be. He was young, not in much work and being served TONS of sweetfeed twice a day. All it took was switching to something without corn or molasses in the first 5 ingredients and the turn around was astonishing. After that, I weaned him off of grain all together. He was in work 4 days a week. On days that he worked, he got a half a scoop of grain as a treat. Without all that extra bulk and fat deposits, he gained muscle quickly and retained it much longer. We upped his hay to compensate for the lack of calories. He was basically a kitten by the time he detoxed from all the grains.