This doesn't seem like it would be that important if you were using pen and paper and designing your forces to play your friends - but what if you were using an App. Now when you added a character to your force, or even a Wish List, you could make sure the right character was leading the right army. or maybe you were trying to add Librarians to a Chapter that doesn't have Librarians (World Eaters for example). Having these Keywords work like metadata or Tags would allow you, as the App User to easily make sure the unit you were building was (a) legal and (b) fluff-worthy.
This seems like a logical idea and frankly how soemthing like an App using metatdata would be ableo to easily identify units and groupings - they certainly couldn't purely by the name! What do you think?
Now lets take a flight of fancy and expand this a bit more and imagine this was then somehow integrated with the online store. The App User could build their force in the App, then click on the miniature they wanted to add and they would be directed to the store to buy it. Games Workshop could then see what people are buying, wanting to field and the cater to that.
If we were GW, this is something we might be looking at!