You can say, it. It helps them keep water in, precents dehydration.
We give our horses salt rocks because it gives them something to do besides eating hay and grass all day
I agree with Tamara , exessif salt licking can be dangerous to the health. Special when a mare is nursing. They can dehydrate. An electrolyte is easier to dose.
It depends on how much work the horse is doing. If it is in high work load and often sweats additional salt into the diet will really benefit your horse. An alternative to a rock salt is adding an electrolyte supplement into your horses feed which has added minerals which is designed to be exactly the same as what horses lose when they sweat. Personally I believe a controlled supplement in a horses diet is better because u can measure how much Salt your horse is getting. But a rock salt is also a very effective way as well and it will most likely be less of an expense. Going back to the question on whether u must supply the salt to your horse, it depends on your horse and the best way to find that out is to talk to a vetanerian.
Depends on your horses' need! If it's food contains all the nutrients it needs, a salt lick is not necessary. If you're not sure, you can always put it up! Make sure you watch horse for the first few days that it doesn't lick/eat it too much, too much salt is bad for the kidneys. Although some horses really don't care for them and won't lick the rock at all... 😅