In the wild horses live in herds for a reason, there is a natural hierarchy that keeps them safer from predators than they could on their own. A lone horse would never be able to sleep or let down its guard long enough to even get a drink without being attacked by predators because it does not have another horse to help watch its back. Eating or drinking puts a horse in a very dangerous position with its head down where it does not have full use of its senses if a predator is near.
Despite previous beliefs on the subject anyone who has spent time observing a herd in its natural setting knows, it is the matriarch mare that leads the herd deciding which areas to graze and when the herd should go to water. The stallion will be the lead once they reach the water and always stands on full alert at the watering hole but most often he actually follows from the back on the way there. The stallions number one job is to keep the mares and foals from wandering too far from the collective mass making it harder for predators to pick off an individual as well as fending off any other stallions or bachelors looking to get a new mare of their own.
Foals born in herds learn proper social manners from birth. More so than it might appear to a person just passing by. Every intentional flick of the tail and ear movement means something. While new foals are fervently protected and kept in the center of the herd at all times, an older foal that is naughty or annoying will often be pushed to the edge of the herd by a higher ranking mare and not allowed back in till the mare decides it has learned its lesson. Young foals show submission by chomping their mouths open and closed.
By learning proper social behavior in a herd, foals are much easier to train and teach to respect people as well. A foal that is raised alone with out older horses to teach it manners is often pushy and harder to train. Some owners are not in a position to have a herd of older horses so may benefit from boarding for a couple months or a year with someone locally who does have a more herd like situation.