There are a lot of different locations in the story but the mountain is essentially the overworld map of Poi's adventure, containing all the different spaces within itself in a way.
In the story, the mountain is the most consistent location, used to tie the story together and represent his internal struggle as a physical challenge, the distant peak of the mountain promising something that he should find within himself. The mountain is a universal symbol for overcoming oneself and has been used in religious texts, mythology, and throughout literature to represent both external and internal challenges.
Ancient Greek mythology uses the trope of the mountain often as both triumph and punishment, like in the myths of Prometheus and Sisyphus.
The mountain also appears in religious texts as a place to become closer to God. Patrick Süskind’s novel Perfume uses the mountain both as an escape from reality and a space to confront oneself.
In Kill Bill Vol.2 the protagonist must climb to the top of a mountain to prove her worth and begin her martial arts training. In Celeste, a recent indie video game, the mountain stands for a distant goal of self-affirmation, and a journey for the protagonist comes to terms with her own issues.
“The spiritual peak and centre of the world, the meeting place of earth and heaven - a symbol of transcendence, eternity, purity, stability, ascent, ambition and challenge (...) Psychologically, climbing the mountain symbolises a great challenge, the stages towards self-knowledge.” - The Watkins Dictionary of Symbols (2008)