Why we need to thin

Folks,
I found a couple of good diagrams that will help illustrate what happens when we don't thin a knife, i.e. this first picture is where the vast majority of knives are at now.

So in the picture above, you can see how repeated sharpening to the primary bevel only (cutting edge) will gradually move that edge up into the belly of knife. Metal is removed when sharpening, it has to be otherwise it is not sharpening. Imagine a 10 or 20 year old knife that has been sharpened a few times or more, the original geometry of the knife is altered and the knife is not performing the way it should or could.

So I thin the knife by removing the metal as depicted in red in the diagram below:

It's a process that just knocks some of shoulder of the bevel off as seen in the bottom of this picture, and  then to maintain the factory geometry or just create a better cutting knife, I keep the knife thin over time as I work on the primary bevel.

Machines don't do this.