It's not entirely a no-no. With history and background, largely its cannon issues. Someone could claim to have once ruled all of TNP thousands of years ago or something and I could either ignore that, or use that as fluff in my own writing.
For example, say I need to write a scene set in some kind of place of learning. I could have my character walk through the halls to their destination, making small talk with another character, who points out a random professor or something who has a theory or has written a book about how an ancient empire once ruled the lands.
It's something I always try to apply to my writing, random stuff that comes up that I can use as fluff is always useful to pad it out. Stops posts that would otherwise be quite short being so.