The abridged summary answer to this question is that it is more comfortable for the dog and you can get the nails shorter and smoother (which is nicer for you). This is actually a much more comfortable method of maintaining nails than clipping them because it does not pinch or hurt the quick. The quick is sensitive living flesh inside your dog's nail. With large dogs, in order to cut through the hard shell nail, you must squeeze the nail between the clipper's two surfaces. If you use a guillotine style, then the nail is pushed hard against the immovable blunt metal while the blade cuts in from one side. If you use the scissors-style, then two blades squeeze in from either side of the nail.   However, neither is very good because when you squeeze in on the nail, you invariably must pinch down on the quick inside the nail. The harder the nail, the more the pressure you must apply. Thus, it can be very uncomfortable for the dog. Also, when clipping on dark nails, you are effectively cutting blind. In order to get the nails back, you keep doing a series of small clips and try not to hit the quick. As a result, you often stop short of where you could go, if you were using a Dremel. When done properly, there is no squeezing or pressure on the quick with the Dremel. Further, you can see "inside" the nail as you gradually grind back to the quick. So, you can judge when you're getting close to nicking it and stop in time. With the Dremel, you can also grind off all around the quick so that it recedes faster and you can get even shorter nails. The closer you can get to the quick, the more you can force it to recede and the more quickly it will recede. Finally, you can grind off all the corners and rough edges leaving nice soft nails that don't gouge nearly so much when your dog paw you for affection.