It’s basically as important as you want it to be as the writer. I’ve done it both ways in scifi and fantasy settings. (And Horror and Modern settings)
Generally speaking, the first time I was really doing the character/background stuff I already realized that some of the “fluff” might bog down the actual story. I basically thought to myself if I was reading this, I might find it mildly distracting to the story text at hand.
So to get around that I would just create a looping “Background link” for further information on such things since I know I’d probably still be willing learn more background content as long as it wasn’t slowing down the story.
Occasionally I also did this for “extra story” content which would flesh out bits of a story, but weren’t immediately involved in the actual main focus.
Recently though in the last couple big stories I did, I didn’t bother with “looping links” at all and just only incorporated background/character stuff into the actual story itself when it seemed important.
Though the reasons for doing it this way were more out of an easier convenience to me than anything else. Just sort of got tired of doing mini-codices.
As far realism is concerned, I just focus more on internal story consistency and try not to worry excessively about realism.