When I was writing Eria, I created a lot of back material for it. For the most part, this comes through as a name here and there, with nothing else mentioned about it, because, quite frankly, there was no need to mention anything else. It wasn't crucial to the story, and, at best, would serve as a distraction, a sidebar, or just something that the reader wouldn't be at all interested in at the time.
It's all there, though. If someone were to ask me "is XYZ 'real', or is it just something you threw in because it sounded cool?" I could say, "yes, it is 'real', and here's some more information...".
Of course 'real' here is a relative term. Like the difference between a real car and a toy car. Both are real, but there is a distinction to me made. You can't drive to the shops and put your groceries in a toy car, but it is real in the sense that it exists.
Likewise, the back material I use is real in the sense that it is written down somewhere for me to reference, change, and otherwise use, but, obviously, not real in the sense that we can go there, meet the people, and so forth, well, any more than we can with fictional material.
I didn't need to create the back material. I could have just 'winged it'. Nobody would really have noticed, known, or probably even cared if they did. I used to do it all the time when I was GMing RPGs, and at the end of the day people still had fun.
So, why didn't I?
The reasons are quite simple.
Firstly, I wanted to write back material. It was fun to do, and I wanted to know more about the characters, locations, and events that I mentioned, even just in passing.
Secondly, it gave me a sense of consistency. If I wanted to check on something, or mention it again, the material was already there, and close at hand.
Lastly, and most importantly, if I wanted to explore the world further, write more about it, and so forth, the material was there.
And, now that I do want to revisit the world of Eria, I'm glad that it is!