While I was mounting the other day, I was advised to give two things to the horse I was mounting, water and salt. The water is somewhat obvious but the salt left me curious, why is that I should give salt?
Salt controls the hydration level of the horse and is generally absent from their normal diet, in comparison to mineral requirements such as potassium, hence the reason you need to provide it. If the sodium level of the horse is below optimal, the horse's body will act to conserve the salt levels in their body at the expense of other minerals and water, leading to dehydration.
Basically, for the horse to maintain a healthy water level, you need to provide sufficient salt to prevent them from shedding too much water to relieve heat in the body.