Ballasts power HID lamps by regulating voltage. They are necessary for HID lamps to function well.
A magnetic ballast is cheaper than a digital one, but it can have problems with overheating and is less energy efficient. Magnetic ballasts are usually heavier and noisier. If you use different lamps, you need to buy a dedicated ballast for each wattage.
Versions without casing come with a cable connector only.