All knife edges become dull with use, even edges on serrated or “never need sharpening” knives. This is because, with use, the edge eventually folds over or breaks when it encounters the resistance of the food being cut or when it comes into contract with a cutting surface.
Edges on conventional, hollow ground knives are weak and become dull much faster than double or triple bevel edges, which are formed by using a 2 or 3 stage knife sharpener.
The images below show what a dull edge looks like on conventionally sharpened and serrated knives.
