Comment by the_af

Good point on Tolkien, though I would argue it's different because the Silmarillion was written for his own satisfaction, not his fans, and actually it was The Lord of the Rings which was initially written as "another Hobbit story" (though the story later "grew in the telling").

I think a more suitable example of what I find annoying is the extra material for the small and big screens, such as The Hobbit movies, suffering from movie elephantiasis; and The Rings of Power, which (over)explain and show the past of characters such as Galadriel, Elrond, and Sauron's rise to power, which in my opinion are best left as the ancient past of LotR (and maybe belong in the Silmarillion, but do note the latter is not as expository as the TV show). And does anybody really think The Hunt for Gollum was necessary?

Yet, if they make them, it's because they are sure fans will lap it up. To be honest I consider this need for endless "filling in the gaps" a sign of immaturity.