So as i said all facing does is move the bb cups closer together.
I am afraid you are off the mark with that comment, its all to do with getting the bearings in line as best you can
This is what park have to say on the matter:
Additionally, some bearing system benefit from having the faces of the shell square to improve bearing adjustment and bearing longevity. If the shell faces are deformed, and are not parallel to one another, the left and right bearing may not be concentric to one another. Machining the shell face improves concentricity.