A blunt or dull knife requires more pressure to cut. This increased pressure plus the inability of the knife to cut into the object increases the likely hood of the knife slipping and cutting a finger or stabbing a hand. A good example is cutting tomatoes. If you try to cut a tomato with a butter knife, good chances are you'll end up with a mushy but not cut tomato. The knife will just slide off the tomato's skin since its not sharp enough to 'bite' into the skin for a proper slice.