Buffing wheels need to be cleaned periodically while polishing due to metal particles and polishing compound that get gummed together on the face of the buffing wheel. This causes many issues while polishing such as scratches, haze & burning. Raking when you notice there is too much shiney build up is usually the sign that you need to rake the buff.
The second reason for raking the buffing wheels is that you need to open the fibers of the treated cloth. When you open the fibers and ends of the buffing wheel, this allows the compound to properly adhere to the material and allow for polishing. The first time you using your wheel, before applying compound, you should first hold the rake up to the buff to "break it in." If this is NOT done, you will risk scratching and burning your knife or work piece.