In the Bleach universe, supernatural spirits and Soul Reapers are the norm, and they need a way to get around. Soul Reapers can use Senkaimon gates, but what about Hollows?
Hollows are the monstrous, masked, eternally hungry souls of the restless undead, and they are often seen hunting human souls right there on Earth. But they are also found in great numbers in Hueco Mundo, the arid world of the Hollows, and one might wonder: how do ordinary Hollows (as opposed to self-aware types like the Espada) go back and forth between these realms?
Opening A Gate
Hollows are born into the world of the living from souls of the restless dead that are bound to the earth by a Soul Chain, unable to move on to the waiting Soul Society. Once that chain breaks, the soul has a hole torn into its chest, and it mutates into a masked, beastly Hollow. These Hollows then roam the earth and devour human souls, even attacking Soul Reapers. For a time, Bleach's story only showed Hollows in this manner, but later on the world of Hueco Mundo was introduced, and it was populated almost exclusively by Hollows and Arrancars. How did they get there? After all, no humans are on Hueco Mundo to die and become Hollows. Put another way, Hollows are not native to the undead desert.
Soul Reapers can freely travel between the Soul Society and the world of the living via Senikaimon gates, which are opened at will. But Hollows are incapable of opening their own Senkaimon gates, and the same is true of Arrancars. Instead, a new technique is demonstrated early in the Arrancar arc: Descorrer. This technique allows a Hollow or Arrancar to open a hole in the fabric of reality, and a tube — known as a Garganta — will connect that portal to the intended destination. Ulquiorra performed this act when he and Yammy retreated from Yoruichi and Kisuke Urahara, activating it by waving his hand. A Garganta was created, and he and Yammy walked right through.
When a powerful Arrancar like Ulquiorra or Grimmjow creates a Garganta, the space inside is solid and easy to walk through, but an ordinary Hollow is much weaker. Most likely, low-level Hollows create tight, unstable Gargantas when they perform Descorrer, and there may even be risks involved. In some circumstances, it may be that a powerful Adjuchas or Vasto Lorde has a retinue of lesser Hollows, and it creates stable Gargantas for the entire party to use.
Why Go To Hueco Mundo?
But another question lingers. Even if low-level Hollows can reliably use Descorrer to travel to and from Hueco Mundo, why would they do it? After all, there aren't any human souls to devour there. One possible answer is that Hueco Mundo is populated almost entirely by powerful, cannibalistic Hollows, and they convene in Hueco Mundo so they can eat each other and determine who's the strongest in a Hollow-only paradise of predators. Since Earth is much bigger and the cannibals would struggle to find other Hollows worth eating, Hueco Mundo would be the best place for this. More importantly, Soul Reapers regularly patrol the world of the living for wayward souls and to slay Hollows. It's tough enough for a cannibal Hollow to survive battles against others of their kind without Soul Reapers getting in the way.
For example, a squad of cannibal Gillians was once roaming Hueco Mundo, seeking to eat enough Hollows to become Vasto Lordes. Grimmjow came across them and, after a brief scuffle, they accepted him as their new leader and protector. Only on Hueco Mundo could they have done this as, in the world of the living, the Soul Reapers would definitely detect that Gillian squad and send powerful Soul Reapers to slay them. But that won't happen on Hueco Mundo, and a Descorrer is the best way to get there.