Why we need MCH bone?

Below picture presentation of question.

Rig Example.blend (605.2 KB)

I see in tutors about rig this decition with MCH bone for creating switchable rotational inheritance on some bone chains. I don’t understand when a MCH bone is needed, and when it is possible to do without it. PierrickPicaut - always use MCH bones when want to create swtchable filter for some transform channel inheritance or just isolate some transform channels from copying by after bone.

P.S. - I would be glad to criticize how clear I am posing the question if something is not clear.

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One thing that came to mind after a test:
yes, you seemingly don’t need a MCH bone… in this particular case. When the Head of one bone is in the exact same location as the tail of the other.
But what happens when the bones are disconnected?

It’s not that you absolutely require it in a simple rig like that, it’s just that it’s a good practice. In more complex rigs you will often layer mechanisms on top of each other and this provides a modicum of separation between them. Also, it ensures the axes are all aligned so the constraints and drivers can be simpler. And finally it will also prevent your mechanism from failing if you need to, say, scale your deformation bones on top of the mechanism.

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I don’t understand how MCH bone helps at situation is where bones are disconnected.

My guess + a hint was given by the artist whose lesson prompted me to ask this question here.

As a child MCH preserves the location in the Source coordinate system. If there is another easy way for this, I don’t know it.

Video - at comment artist that inspire me ask this question answered me about Local with parent orientation mode. I conveyed the meaning of his words in a picture.