What is the purpose of the battery that is used in motorbikes?
The primary purpose of a battery on a bike is to get it started and to power the electrical components on it. If your bike has a kick start, then the battery is not strictly necessary as the kick start can get it started and the alternator can supply power to the lights while the motor is running. The battery is still necessary though as there may be times you want the electrical components to work without the bike being switched on. Also if the alternator fails, the bike will be dead. I watched an episode of ice road truckers where one of the trucks lost their battery during a trip and the driver did not notice till he got into the depot as his truck was running perfectly fine until he stopped the engine.