Every sharp knife sharpener knows the importance of washing a knife after sharpening it. Without doing so, the small pieces of metal broken off the blade during sharpening can easily and unknowingly be transferred to your cutting board or whatever ingredients you are about to chop. This can be particularly problematic when slicing something like cheese, as you do not want sharp metal particles to be incorporated into your food. Careful cleaning after sharpening is thus essential in order to ensure that all food remains bacteria-free, as well as contamination-free from sharpened metal fragments.
And this video is beautiful proof of why you should clean your knife after sharpening! You can easily see iron fillings on your cheese after cutting it if you do not clean your knife after sharpening.