That is for controlling the headlights when they are directly wired to the battery. Some have complained of dim headlights. The problem could just be the bulb connector or dirty battery connections. I would not do this, I would just try cleaning the electrical connections and using a new bulb.
If still not satisfactory, try E-code (european headlights), or an HID retrofit from the Euro cars. Don't bother getting an aftermarket HID kit since they don't have the right color range, are mostly not high quality, and aren't designed to work with the stock reflector headlights. You need to change the housing for an HID bulb kit to work correctly.
The problem is often loose or bad headlight connectors. There can be a voltage drop to the headlight bulbs, making them dim. If you add HIDs, you may also have to do some wiring work. IMO, get an HID headlight, for all the work just go all the way.
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