Characteristic Twins p.18 Players Guide
A GM could use it as a quick guide to NPC personality when just picking up the NPC card and not having much else to go on, E.G., best stat is Str so pick one of those personality traits.
It is useful for GMs, especially new ones (or GMs new to WFRP) to allocate skill/characteristic checks for various tasks that might not be inherently obvious.
As another example, I GM could have a scenario where the PCs are roleplaying a dream sequence. Combat in the dream could potentially utilize Int/WP/Fel instead of the physical characteristics. Etc.
I understood it as a way of expressing some of the concepts. TO/WP are both used to resist things, Str is used for damag with physical attacks while Int is often used in calculating spell damage. Agil/Fel are both used for initiative. I think they just function in similar ways allowing the GM to utilize his understanding of the physical characteristics to overlap the mental characteristics. I think it is ment as a logical step.
Just my 2 cents