Yes. It helps them stay hydrated
In fact It's advisable to make permanently available a rock of salt, for multiple reasons :
Body Needs in salt are very variable, including for an individual (especially according to the sweating).
Frequently, the concentration of salt in food is limited not to disturb the palatability of the ration.
Given that the horse regulating itself the salt, the availability of a rock of salt securise dietary intake.
Have a great day,
The Destrier Team.