Could be to replace beneficial bacteria or to get some nutrient missing in the diet. Any signs of ulcers? Any course of antibiotics done lately? I would be feeding trace minerals as suggested. Know that horses are grazing animals. Unlike us, they are producing stomach acid 24/7. If there is no hay going through and the horse is waiting for his next meal, then the stomach is eating itself while he waits. Slow feeders are also a good regulator and fight boredom. Salt and mineral blocks don't cut it and neither yogurt. A proper probiotic is a good addition to any diet.