First, I have to say I have not been here in a long time so this may be old news but IMHO, the single biggest thing GW has done is push the non-game content. Way back in the day (80's and 90's) there really wasn't any fiction outside the fluff in the rules. Once they decided to start pushing out books through the Black Library they solidified the background universe.
If they hook someone through the fiction, it is relatively easy for that person to jump into the games and understand who is who. Or, it was. I have not looked at WFB for years so I don't really know much about AOS but (again) in the past if you were to randomly pick up a William King book (Gotrek and Felix stories) you could easily make the jump to WFB and know the layout. The same goes for the 30K and 40K stories.
I started playing with Adeptus Titanic and Space Marine a very long time ago and we were starved for understanding the backstory. There were a few books around but GW had a tight fist on fiction and really never pushed the medium outside the games.
Of course, the downside is they started pimping high cost models through the fiction. So that didn't necessarily end well...
FWIW
DLM