could you do me a favour and can explain a bit deeper why "Shift w" - "Deform" - "Disable" is necessary? It doesn't make much sense to me....
thank you
Only bones that have the "deform" option enabled are able to deform bones with an armature modifier - as long as there is a vertex group on the mesh with the same exact name.
You technically can leave them enabled on any bone as an armature modifier will only work if there is a corresponding vert group, however, this means you have to be really careful when using auto weights as it will create vertex groups and weight them for all the bones with this option enabled.
Turning it off for all except deformation bones will cause you less headaches in the long run and makes for smoother/easier workflows.