There were four horses used by knights, the destrier, courser, palfrey and the packhorse. They were each used for their own reasons, during the Middle Ages.
Destriers were used by knights as warhorses, as they were large, powerful stallions. It carried knights to battles, tournaments, and jousts. Destriers were not a breed, but a type of horse. They were the strongest warhorse. In battles, the destriers would wear a shaffron, a head piece for protection and a crinet, the metal plates that covered the neck. It was a very expensive horse.
Another warhorse, yet less valuable, was the courser. It was a fast and strong horse. They were more common than the destrier. It was popular with nobles, ladies, and high-ranked knights. As well as being used for riding, it was also used for hunting and ceremonial uses.
Knights who travelled around had packhorses with them to carry luggage. Packhorses were used when crossing rough terrain to carry many things, including goods. It was very useful to the knights, leaving the other horses to do their normal jobs, riding, hunting and fighting.
Those horses were used by many knights. The knights needed a lot of money for the expensive and valuable horses. The most valuable would probably be the warhorses, because the knights depended on the horse to take them to battle.