Vitamin K is one of the fat-soluble vitamins, meaning it can be stored up in the body.
The main function of this vitamin is for blood clotting to occur, which we all know is critical to our accident-prone horses!
The one use that most people will ever personally see Vitamin K administered for, is if the family dog happens to ingest rat poison, at which point the dog will receive an injection of Vitamin K from the vet. Rat poison functions by limiting the clotting ability of the blood, thus basically causing internal bleeding and eventual death of the animal. Vitamin K injections help restore the clotting ability, hopefully in time to reverse any damage.
In horses, forage sources and the bacterial activity in the gut upon ingestion of adequate forage produces enough Vitamin K, and thus it is not generally supplemented in the diet. Toxicities and deficiencies can occur, but are very rare.