If a horse's tooth is continually erupting, what is the point of filling a tooth, won't a decayed area just grow out?

No, even though the tooth does continue to erupt for the majority of the horse's life, the decay seems to invade the tooth faster than the tooth erupts. If the decay is coming close to (determined by depth of dental probe) the pulp cavity or is in the pulp cavity, the decay will do much more harm in a shorter time than it would take for the tooth to grow out. That type of decay is very painful and can cause abscesses. The tooth will be lost eventually. Whenever the teeth are out of position for any reason, be it a natural configuration of their mouth, or a missing tooth, the mouth as a whole doesn't wear correctly. Then it becomes even more important to maintain the teeth in proper wear so further damage isn't done.  more