The Veil is a spell that was constructed to keep out those who would kill and eliminate the fae outside its borders, and to repel those who fell outside of Mundane reality through a pill or a bottle or a smoke with no training and guidance on their own side. It was concocted by all the fae who could see the value of protection and who wanted to live in a certain amount of peace in their long lives.
As the Veil and the spell that sustains it unravel, more and more of the denizens of Fiction, the HyperDimension, Cyberspace, and even Mundane will be coming to call. The Twilight Lounge may become very crowded, especially if Belle is not there to ride herd on the cats that hang out there. The Twilight Lounge is not properly part of Faery or any of the other dimensions, but is a portal, a nexxus for moving from one to the other, and possibly other destinations.
The Fae occasionally brought someone across, or went slumming themselves, but for the most part, the Veil between Mundane, HD and Faery was solid and useful. Some of the Fae stayed too long in Mundane, and died, but others found a way to live here permanently, in a pocket world called Mundance–a muse-ical typo that has happened enough times to make it come true. From my perspective, all the urban fantasy genre comes from Mundance–a little bit of Faery encapsulated in the fossilized amber of Mundane. What happens if the Veil completely unravels remains to be seen.