Many problems with the digestion system are a result of beneficial microflora being out of balance. When that happens, the digestive tract can be overrun with many foreign organisms and cause diarrhea which leads to dehydration and malnutrition.
When a chick is born in nature, its digestive tract is sterile. So, chicks actually get this intestinal microflora by eating some of their mother’s fecal matter. The microflora is deposited with everything else and when consumed by the chick, it allows them to culture their own.
In a controlled environment where the chicks are not raised by their mothers, probiotics can accomplish the same goal.