The Null Void is often utilised by the Plumbers, especially on worlds with no official relationship with the Plumbers such as the Earth prior to Ben finding the Omnitrix. Plumbers possess Null Void Projectors that create portals into and out of the dimension.
The Null Void can also be accessed by means of magic spells.
There are naturally occurring 'soft spots' in the multiverse that lead to the Null Void, however they don't often lead back out.
Although the Null Void is mostly inhabited by prisoners, there are also some beings that were born here or evolved here. There are small pocket villages of settlers that have lived here for generations, but most people born in the Null Void are the descendants of prisoners that either could not escape or chose to make a living here.