Not used on rally cars or even sprint or hillclimb cars at one point, don't know if the rules have changed for sprints. Basically there was one set of rules for competing on the same track or competing against another competitor, as in racing or rallycross. If you break the light during a race there is no chance of picking up the pieces and clearing the track. But in events against the clock, the run can be stopped and the track cleared and a rerun given.
These rules also applied to medical certificates as you needed one to compete directly against other competitors, but not for speed events.