Why do horses have salt licks?

If you look in the stalls at a barn you might see that at least some of the horses have big blocks of either white salt or a red substance in their stalls.

Horses are given salt licks for two reasons. The first is that they're a good way to get minerals into horses, who tend to consume just as much of a lick as they feel they need (they're pretty good that way).

The second is that having a lick helps keep a stalled horse from getting bored - it's kind of a toy for them.