When we are thirsty, we drink. A horse needs a trigger to drink, and if it doesn´t receive that trigger it may well end up dehydrated. The one mineral a horse´s system does not tell them they are short of is salt. If they are dehydrated they will not seek it out to resolve this, setting off the trigger to drink. So providing a natural salt rock, or crushed salt ensures they drink regularly.
Do not be fooled that your man-made mineral block is going to provide all the minerals your horse needs. Take a look at the label. It probably contains only 6 or 7 minerals. Horses often also don´t like them. They can be very fussy about these things. Just because you put a salt block out for them, don´t think you are done and dusted!
Many of us have horses training for tough events, like endurance and cross-country. This demands a lot from them and, with the additional heat of summer sun, will drain them of electrolytes. Unlike the tongue of a cow or cat, a horse can not lick all the minerals he needs from a block without his tongue getting sore. A horse´s tongue is very sensitive (also the reason we don´t use bits!). So the addition of natural crushed rock salt in his daily feed (a good tablespoon full) will maintain good levels of hydration. The salt assures they will drink.