All of the tooth structure that an equine will have is present at birth. In addtion, an equine's upper jaw is approximately 30% wider than it's lower jaw . As equines age, their teeth erupt and the difference in width between the upper and lower jaw results in the arcade (the surface of the teeth) being unmatched. When an equine chews in what could be described as a figure-eight motion, uneven wear results on the arcade which causes sharps points on the edges of the molars, creating the need for dental floating.
Note: Equines' teeth do not grow... growth implies regeneration and their teeth do not regenerate. Once tooth structure is gone, it's gone permanently.
Notice how far the molars extend into the upper and lower jaws. Given this finite length of tooth that a horse has for the duration of its lifetime, proper dental care is critical to maintain the integrity of that structure for as long as possible.
For more details about equine dental anatomy, please see this page: