If a knife is not sharp enough to cut what you need to cut then it is useless. The edges have to be sharp throughout the life of the knife, but it is not so. If you see the cutting edge of a knife closely with the help of a magnifying glass or a microscope, you could see there are small teeth that make the look of a saw. This is what makes the knife sharp and useful. On using a knife repeatedly it becomes blunt. If you look at a blunt knife with a magnifying glass you could see that the saw like cutting edges are bent and not in alignment. You have to make them straight to be useful again.