You are correct we never see it do anything particularly interesting, but on-lookers specifically state it warps space. How they made that determination is anyone's guess.
"Just condensed spirit energy" means nothing. It's like saying a Naruto attack is "just condensed chakra". Or that Might Guy's attacks are "just raw physical force" - a particularly apt comparison because Might Guy does bend space, according to Madara, when he opens the 8th Gate.
The actual process goes Earth <---> Void <---> Hueco Mundo, with Void <---> Dangai and Void <---> Soul Society also shown. Furthermore, Hollows aren't limited to single location at any end. We've seen Grimmjow leave and arrive at multiple places. So even if he has to take a detour through Hueco Mundo to get to a different place on Earth, the ultimate effect is similar to space-time-ninjutsu.Originally Posted by Razade