Horses are plant eaters. Grasses and other plants have tough structures, which makes them difficult digest and retrieve energy from. It is important to chew the plants properly and grind them fine particles in order to get most energy out of them.
Us humans chew by lowering and lifting our lower jaw, but horses chew by making a lateral, or sideways, movement. Us humans can hardly make this movement, because our lower jaw 'slots' into the upper jaw. In horses, however, the rough surface of the molars serves as a grinding plate for tearing and shearing the fibruous material.
Chewing the tough plants by grinding the teeth's surfaces makes the molars wear down with eating. This is compensated for by the continuous growth of horse teeth. This continuous growth of horse teeth means that they need to be worn down appropriately.
The more fibrous the feed, the larger the chewing movement needs to be. So chewing movements for fibruous roughage are big, whereas chewing movements for less fibrous material such as concentrates are small. The large chewing movement wears down the entire grinding surface of the molars evenly. In contrast, small chewing movements wear down only part of the griding surface of the molars.
Wearing down only part of the grinding surface leads to deformed molars. The part that is not worn down continues to grow, creating sharp edges on the molars. On the molars in the very back of a horse's mouth these edges can develop into big 'hooks', limiting the motion of the jaws even further, making the problem progressively worse. Sharp edges and hooks result in damage to cheeks and tongue and limited jaw movement. This often shows as excess salivation when a horse has a bit in his/her mouth, refusing the bit or when a horse becomes a 'messy eater', dropping feed from inside the mouth. It may be that you horse does not show any of these signs, but still suffers from a painful denture.
Feeding adequate amounts of roughage helps to prevent sharp edges and hooks on horse teeth. Roughage is not a cure, nor a guarantee against the development of sharp edges and hooks, but feeding inadequate amounts is bound to result in problems.