The Soul Society in Bleach is a resting place for the dead, but Hueco Mundo is for the restless dead. Hueco Mundo is cruel and unforgiving, and there is no path to salvation. Once someone is stuck there, there is no other option but to accept the harsh rule that you either kill or get killed there. When Ichigo and his companions were looking for Orihime Inoue, they realized that Hueco Mundo was no place for ordinary men. One can’t win anything by being there; one either survives or gets killed; that’s all there is to live in the “hollow world.”
The place of the restless dead has nothing but death and despair for its inhabitants. Therefore, Hueco Mundo is nothing but a desert of endless white sands. The sky above is starless and depressing. The hollow world’s vital spiritual energy ensures that the Hollows are nourished well enough and don’t need to feed on each other to survive. As far as transportation is concerned, one has to either fly, slither or run since there is no infrastructure outside Las Noches. However, movements from one place to another for the Hollows or the Arrancars are easy to use Garganta portals.
Although Hueco Mundo did not play a significant part in the early story, it became quite important in the later parts, especially the Arrancar arc. Its significance increased after Rukia was rescued and Ichigo had to master his inner Hollow. If we go chronologically, Hueco Mundo began when Vasto Lorde Barragan successfully took over the entire world and created an army of Hollows. Although it might seem that his accomplishments might have made him incredibly happy but by the time he achieved all of this, he was already bored, and everything seemed futile to him. Even after winning the game in Hueco Mundo, he was not happy. Soon, Sosuke Aizen took over, and he became Espada #2.
Grimmjow Jeagerjaques, too, had a somewhat similar experience. He ultimately realized the futility of his quest, just like Barragan. He just simply swallowed everyone who came in the way until there was no one else except him. Hueco Mundo eventually became Hell, a place with no salvation, only misery, and violence for its inhabitants.
When Aizen was toppled, The Wandenreich muscled in, and Yhwach’s army murdered Hollows. Tier Halibel, the queen of Hueco Mundo, was captured and defeated. However, once the Wandenreich was defeated, Hueco Mundo must have returned to its natural state. Hueco Mundo gave its inhabitants very few options; one either won the game of survival or died. Even the ones who conquered Hueco Mundo realized the futility of all of their efforts.
Source: https://a6office.co.uk/bleach-the-harsh-and-unforgiving-conditions-of-hueco-mundo/
