User blog comment:Fusedtwilight/SEASON 8!!!!!!/@comment-5115351-20120522135001/@comment-4807493-20120522192103

Purgatory in the show verse has been slated as the 'resting' grounds for those souls that have been tainted by Eve and her children, or creatures banished there by God in the past. If you were to look at it from a lore perspective however, there's a few differences. It's more like 'limbo'; neither of Earth or the afterlife, but a place in between.

Using that logic, you could say that those ghosts who aren't guided by reapers could go there when their ties to Earth are destroyed, and await some entity or force to move them on from there. Alternatively, it could just be that having those ties cut means that they skip the middleman altogether, and get a oneway ticket to heaven/hell.

While I'd love it if they full on explored those questions, I honestly think that it probably will never be explained. Adds to the whole 'mystery and intrigue' of the show, and all that.