Equestrian arenas for practice or competitive performance require almost constant upkeep to avoid causing harm to any horse, rider, trainer or anyone else. A big part of this is the surface within the arena area, which is difficult to maintain it consistently uniform, not any harder or softer in different spots. The nature of the material normally used, preferably silica sand, is that organics get their way in, along with dust and dirt, that can become compacted with exposure to moisture, drying into large and hard clods. These can shift or turn when a horse steps upon them, creating instability and an unsafe situation for horse and rider.
By blending in unpainted rubber mulch in 50 / 50 equal amounts with sand, the nuggets prevent the sand from ever getting to clumping up, so it's easier to create and maintain that ideal consistent surface that every equestrian deserves.
Horses and their riders have to work together in harmony during performances or competitions, much more so for jumping, dressage or anything requiring agility and responsiveness at a much higher level. If an inconsistent surface caused a horse to lose it's footing at any point, it could result in serious injury to the horse and rider. Injuries can range from minor to major, temporary with a good prospect of recovery, or permanent limitations, and with some it's final.