For auto transmission, a computer would shift the gear at normal driving situation according to your speed. Gear 4,3,2,L is mainly used for engine brake. When you driving down hill for long time, you should shift to 2 or 3, and don’t give gas. Slower engine rpm acting as brake to slow down the car. You should not use regular brake; it would be over heating in consistent braking. . Another situation is pulling a trailer or other heavy load. You shift to lower gear to have more power.