Basically, a point light source is needed to generate the image, so this is why HID bulbs are used. You need a reflector to use as much of the wasted light as you can for maximum brightness, and to maintain this point light source you need it to be in the middle of a sphere. The focal point of a sphere is the very center, so if you put the bulb part in the very middle of the sphere the light will bounce off of the reflector and back into the bulb. It is hard to describe, so look at the pic:
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