Because it's fun, it's useful, and in some cases, it's required (like for certain horse riding competitions). Originally, horse's manes were braided as a safety measure: simple braids kept the thick manes of working horses out of harnesses and equipment. In the military, horses manes were braided for reasons of both function (for example, not impairing a soldier's ability to reach for a weapon) and form (creating a formal, organized, and impressive-looking army was part of good warfare). Today, horse braiding is part tradition- because we've done it this way for so long- and part fashion. Today more than ever, creative braiding is welcomed in the show ring and still serves the functions of keeping manes free from interfering with reins and harness, helping keep horses cool, and stimulating mane growth.