When you apply a sharp knife to the surface of a tomato, cucumber, carrot or other food item it should - if you are holding it firmly and applying minimal pressure - sink into the particular item without effort. As such all you really need to work on is your technique and making sure you keep your fingers out of the way. With a dull knife however things aren’t quite so simple. When you apply a dull blade to, say, a tomato it’s not going to slice into the skin. Instead the skin will be able to push back meaning you’ll need to apply ever more pressure to get the knife to penetrate. Increasing the pressure on the knife increases the likelihood of the knife slipping off the offending food item to one side or another. And if it slips to the side where your hand is attempting to hold the vegetable steady you could be in store for a very nasty cut. Because even a dull knife will cause injury if there’s enough force behind it.
Now you may wonder, if a finer stone produces a better cutting edge why do I need a coarse stone at all? The answer is that finer stones work much more slowly than coarse ones. The amount of metal a fine stone removes with each pass is much less than what a coarse stone removes. If you have an edge with a good shape to it from a coarse stone, this is not a problem because the fine stone doesn’t have to remove much metal to improve the edge. If the edge is dull, it will take many times longer to reshape it with a fine stone than it would with a coarse one.