Disc brake cleaner works like this: when you hit the brakes, the calipers grab the rotors and halt the vehicle that is equipped with disc brakes. After grabbing, the material on the calipers vaporizes a bit and becomes dust. In turn, the dust coats the front wheels with a fine black oxidized powder. As the brakes are continually used, the coating gets thicker and the eventual buildup may damage the rotors. Disc brake cleaner not only cleans up your front wheels and lubricates them, but it also cleans away any residual buildup on the calipers and rotors and keeps the system working normally. Disc brake cleaner may also hold off the need for a re-facing of the rotors for a time.
For more info, see 4 Must Have Brake Repair Tools