When manoeuvring. Because after starting a cold engine the engine revs may rise enough to engage drive and if you are not prepared for that the car could travel 10 feet or more before you can get your foot from accelerator to brake and could crash the car or run someone over. Also, if you were to panic and stamp your right foot down on the brake you might miss and hit the accelerator instead, as usually happens in 'unintentional acceleration syndrome' death crashes. There is need to left-foot brake all the time, though, once you get the knack, it will help you to drive more smoothly and keep your automatic fully under control at all times. In fact, in a wet braking test in Germany I stopped a Mercedes from 80km between 4 metres and 5.5 metres shorter by left foot braking than I could right foot braking.