Manga covers are usually created by a combination of artistic skills and design concepts. Artists start with an idea or theme for the cover and then sketch it out.
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.
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.