It contributes by expanding the world - view. The 'Made in Abyss' series has a vast and complex world, and 'Old Stories' can show different aspects of it. It can introduce new characters or events that are related to the main story, thus enriching the overall narrative. It might also give a better understanding of the history of the Abyss, which is crucial for a full appreciation of the main series.
We can expect unique world - building. It likely delves deeper into the lore of the 'Made in Abyss' universe. Maybe we'll get to know more about the origins of the Abyss and the strange creatures within it.
There might be a fanfic that focuses on the backstory of one of the characters, like Riko. It could explore her early days in the orphanage, her training, and how she developed her strong will to descend into the Abyss. This could give readers a deeper understanding of her character.
The answer is yes. Made in Abyss began as a novel. This original form allowed the creator to lay out the core ideas and storylines that were later brought to life in other visual and narrative formats.
The Made in Abyss manga is on hiatus for now. Maybe the author needs time to flesh out the story or there could be other behind-the-scenes factors affecting the schedule.