One of the great things about playing roleplaying games with a strong historical context is you get to buy books outside the hobby to support your creativity. I got the above three books to support the Adventurer Conqueror King System (ACKS), and they are wonderful reference guides on how the era is supposed to look, feel, and act. While I am not playing in Middle Ages Europe for my game, the conflicts, people, and struggles of the time directly translate into in-game inspiration and background. I could flip to any page of these books and instantly have an adventure seed.
There are also conflicts baked into these books, like the Magic in the Middle Ages book discussing the history of magic, the church, and the early laws against witchcraft. If you have a civilization where the power lies solely with the church and not the state, this stuff is important. A monastery is established in a remote region, and the church moves settlers in. A town grows in the area, and the land begins to be tamed. And then mages move in, or magical artifacts are found.
If this were D&D, oh, that is another character class, someone hire an arcane caster, please? Character classes don't have reasons to mistrust each other! What is this? Somebody remind everyone of party unity please and play nice with each other. That "modern fantasy" feeling hits and people start behaving as the rules tell them to.
In a more grounded and historical game, the church may keep a close eye on mages and limit the practice of magic. They may ban it, or even hunt practitioners, depending on the faith. Of course, as characters get higher levels and begin to start their own kingdoms, they can set any rules they want. My reading of this book gives me ideas for the game outside the game's library of sources.
And of course, you get the novels and game worlds of the official game, and if you tried to bring in ideas from history into "a fantasy world" you may be called out for breaking what one book said, changing the lore of the world, or stepping on someone's expectations. You can't make druids distrusted! I play a druid! That would never happen in the Forgotten Realms! There are too many powerful mages in this world for that to happen!
The fact a lot of "in-game lore" and the rules themselves contradict history and have to be argued for is a very strange thing. It is as if somehow we worship this idolatry of a "generic fantasy world" so strongly that actual things that happened in history are somehow invalid ideas.
And this "generic fantasy world" becomes a Wonderland of silly rules and contradictions, things that can and cannot happen, words that can and cannot be said, codes of conduct and manners of dress, and concepts and injustices being brushed away and everyone being told to forget about them because they didn't happen.
Back in the Middle Ages, peasants and serfs were practically owned by the lord of the land and needed to be given permission to travel. There was no modern cosmopolitan society, no colleges, few organized guilds, no merchant class, no wealthy class, no intellectuals, no suburban life, and governments were weak compared to the almighty church. Families gave children to the church for blessings and benefits. This is the starting point, if players want to change this they are free to do so.
If I started with D&D, C&C, B/X, or any other game that feels more "modern fantasy" I am not getting these concepts. The books feel strange. A lot of what is in these books is out of place in the D&D cosmology or Pathfinder world, and parts of them are banned outright in the lore.
And I like collecting books. I do not have to rely on a game company's release schedule to find inspiration on my favorite topic or style of the game world. I have all of Amazon at my fingertips. I am not limited to one source, company, view, content guideline, or pigeonholed into the "fantasy world" box when I am shopping for my library. I am also not tied to web portals or having my library taken away by policy changes, business closings, or license invalidation.
The company can stop printing the game and I will still have near-infinite sources of content. This is one of the strengths of B/X and the OSR as well, none of my B/X purchases are invalidated by a new version of the game, switching from Labyrinth Lord to OSE, playing ACKS or C&C, doing LotFP or AD&D (or ADD&D), S&W or any other B/X game.
I have the freedom to choose.