Start-stop system is designed to improve fuel efficiency in city driving when you spend a lot of time standing still at the traffic light. It shuts down your car and restarts it again automatically when you press the throttle pedal.
However, many people find it too intrusive and annoying and in many cars it cannot be deactivated. Furthermore, in some applications the restart is too slow, which is frustrating even in everyday driving conditions.
Lastly, if you race, your performance is greatly hampered by the start-stop system. It will increase reaction and revving up times and thus slow you down. This is why start-stop deactivation is a must for all racers. No performance vehicle should be hampered by it.