The battery acts as a store of energy. The bike generates its electricity much the same as a car does. The battery fills the gaps in. When you change gear, stop start traffic riding at night. These are times when the engine cannot make sufficient electrical energy efficiently. You have to remember that bikes don't have massive charging systems as they cost horsepower. 125's don't have anything to spare. Would you give up 20% of your power for brighter indicators, and carry another 5kg of battery and the associated cosmetic implications? didn't think so. Because of that the bike relies on the battery more than a car does, without the battery the lights and such will act as you say, but! without the battery to stabilise the electrical system there will also be voltage spikes when you have that red mist moment at 10000rm and you electrical system will cop for 15v and then you blow bulbs.