A ballast’s purpose is to start and control the energy flow through a lamp. It regulates the electrical current that flows through the HID lighting so that they work efficiently. This is particularly important in indoor gardening where light is limited and artificial light needs to be introduced. A sufficient electrical current controlled by the ballasts so that the bulb won’t be destroyed.
Design complexity varies in ballasts. The simplest form can be a series resistor/inductor, capacitors, or a combination of the two. For more complex ballasts there are electronic versions that are used with fluorescent lamps and high-intensity discharge lamps. An electronic version is intended to limit the current through the lamp tube. The tube has negative resistance characteristics and without limiting the current it would increase to damaging levels.