From what I could tell, a Hub World serves 2 purposes:

1: Many miscellaneous items such as Pistols, Jackets, Lizards, Rocks, etc. land on the Hub World, so it is essentially the universal garbage bin, although most items that appear there are in at least partial working condition.

2: The Hub World connects other universes to each other. The 2 smartest people in the universe developed a device allowing you to choose which universe you want to go to. They placed 1 of these devices on every planet in every universe. Using the device on another planet will take you from that universe to this Hub World to the universe you want to go to. However, the Universal Transporter System, as I call it, has been shut down following the Trocasian's invasion.