The main purpose of the ballast is as a current limiting device. When the light is off there is no physical connection between the electrodes in the bulb. To strike an arc takes a couple kV as an earlier poster said. Once a plasma is established (i.e. bulb is producing light) to all intents and purposes there is now a 'connection' between the electrodes. Without a ballast this is basically a short circuit and the current will keep trying to increase and increase - eventually knackering the battery, bulb and wiring. The ballast controls the current and keeps it steady. The simplest type of ballast is a resistor - but they are very inefficient and get very hot. The ballasts in these types of light are electronic ones IIRC and are a lot more efficient....
Ballast is specific to the type and rating of the bulb - so you can't swap to LED and use the same ballast as for HID. Do LED even use ballast....?
Right.....enough of that. Off to the pub for lunch and then drive to Weymouth
Rob