Spoilers up to
Supernatural 6.07 and Massive Speculation. Also blithering.
Note: For the purposes of this post, the definition of Purgatory is as given in the episode. No implications toward real world beliefs or definitions are intended.
Crowley's possible tactics
Purgatory as middle ground. Seems important tactical territory. Sensible in an unstable situation where Heaven's warring with itself, and Hell's got a shiny new Overlord, and monsters on the mortal plane are breaking he rules and kitting up for battle that securing the middle ground would be a tactically wise maneuver, both from a defensive standpoint, and a 'planning to launch an attack' standpoint.
Not only that, but given that it's full of the souls of 'every dark thing' that's ever died, ever, does it not seem like the denizens would be quite happy with the chance to bust out and wreak havoc somewhere? And I would guess that Crowley, as King of the Crossroads has some experience in herding souls and may figure his resources are up to riding herd on some new recruits/conscripts.
Plus having control of what's being presented as the monsters' afterlife in the world of Supernatural might be usable as an effective deterrent or bargaining chip in getting the monsters either on-side or at least held back in a kind of cold war position.
In any event, I would guess that Crowley is not acting defensively here, he's attempting to overthrow Heaven while it's in internal turmoil (which would be a tactically sound decision from his perspective).
So as far as the season arc goes, guys? Shit just got real.
Finding Purgatory
If you were asked where your soul would be going when you die, regardless of where that may be, would you be able to give directions to get there that weren't dependent on the person you are speaking to following your belief system before they died themselves? Likely not.
I'm thinking the Alpha has been kidnapped so they can kill him, and let a Reaper collect him, and somehow use that Reaper as a portal to Purgatory. (What's Tessa doing these days...?)
Although he did imply he knew where it was. Hm. Bears more pondering.
Stopping Crowley.
Is likely going to involve re-stabilising Heaven, which may involve human souls who've gone there, especially since this seems to be the season of the soul, with the stealth Big Bad as Crowley former King of the Crossroads and chief soul collector, Sam's missing soul, Bobby's restored soul, souls having an as-yet-undisclosed value as mentioned by Balthazar.
The Heavenly Roadhouse has to be involved. Ash and Co. are the only ones we know of who've figured out how to slip between people's personal Heavens. Plus that whole soulmate thing of Sam and Dean's has to come up again and get used for some purpose.
Anyway, just a few random bits of blithering.
NO SPOILERS OR REFERENCES TO PROMO MATERIAL OF ANY KIND IN COMMENTS PLEASE.