Basics of Equine Dentistry
A horse should get a check up every 6 months.
Veterinarian vs. Equine Dental Specialist
A vet usually doesn't have all the equipment to be totally thorough and may miss things, especially in the molars, due to the lack of specialist equipment; the specialist should have the equipment. Not all vets are certified to do this (their basic area of specialty isn't this specific); the specialist specializes in the equine dentistry and they spend most of their time doing the dentisry stuff, as opposed to spreading their practice to several more general areas.
The Bad Stuff
Enamal points, rough edges, hooks.
All the bad things prevent good chewing, so a horse doesn't get all the nutrients from it's food. As a result the animal may drop weight and lose condition. This means the owner has wasted alot of money on feed that couldn't be utilized. It is possible that if a horse has problems that you get corrected that the animal won't need as much food to keep it's weight at the level you want it.