If a serious incident occurs, the racing needs to stop immediately for the safety of all competitors, marshals, spectators and emergency services. There could be marshals and medics on the track attending to both riders and debris. There may well be the need to get emergency vehicles onto the course. A red flag can’t stop the initial incident from happening, obviously, but stopping everyone immediately could prevent the incident becoming much more serious when a rider barrels into an unforeseen situation at 200mph.