We use a system (Tormek) that sharpens using a whetstone (a semi-porus sharpening stone) that sits in a bath of water - but why is that better?
Sometimes you see knife sharpeners post pictures of them sharpening a knife on a belt sander - exciting sparks are flying and it looks cool... but it's really not. There are two main reasons we don't use this method when sharpening as a general rule:
1. Heat - those sparks you see and the noise you hear when "dry grinding" are really bad. Let me explain. When knives are made they start as a unhardened steel so they can be worked efficiently - this is the state steel want's to be in. They are then "hardened" using a very specific heat treating process - this gets them really hard but with hardness comes some brittleness, so they are often then "tempered" at a very specific temperature to bring them to the exact hardness the manufacturer wants - less hard and less brittle than the initial hardening. If you continue to add heat to the knife it will eventually return towards the soft state of the original material.
Dry grinding adds heat to the very spot that you least want it at the microscopic level... the edge of your knife and even worse at the apex of the edge. This leads to reduced edge retention and the need to get your knives sharpened more often. We only dry grind when we need to do major repair... we always do a final sharpening wet.
2. Safety - Dry grinding releases particles from the knife and the abrasive of the belt into the air. Even with good ventilation it is my opinion that breathing these over long periods of time exposes sharpeners to harmful toxins. Anecdotally, I've known more than one sharpener that has had some form of cancer late in their careers. I can't prove these health issues are because of dry grinding, but I suspect it has some bearing on our health and I'd rather not risk it to save a few minutes while sharpening.
We primarily use wet sharpening techniques which eliminate the heat and particulate issues. If you are working with a sharpener be sure to ask them how they sharpen (dry or wet) and consider what is best for both the sharpeners and your knives!
-Vijay