The Center of the Multiverse
It's usually upsetting to Prime Material bashers when they hear that their little world isn't the center of the universe, which is why they're known to planars as the Clueless. Members of a faction called the Signers might argue otherwise, but smart folks say there's no particular center to the planar multiverse. Rather, it all depends on where you stand. Folks in Sigil see the City of Doors as the center of the multiverse, folks on the Prime Material Plane say their own worlds are the center of the universe, and the efreet brag that the City of Brass is the center of it all. The thing is, maybe they're all right and maybe they're all wrong.
Maybe they're all right because - the multiverse being infinite by most standards - no matter where you stand, that's the center of all things. The Signers have turned that idea into a whole philosophy: "I'm always at the center of the multiverse; therefore, I must be the center of all universes," they say. 'Course, the Signer's aren't quite right, because by that logic everyone stands at the center of the multiverse. (The Signers resolwe this little paradox by ignoring it.)
In blunt words, the fact is that there ain't any place in the whole multiverse that's more important than any other. For instance, Mystara on the Prime Material Plane is not the most powerful, influential, or important point of the multiverse; it's not the sole reason all the other planes and powers exist. Hey, the uncounted layers of the Abyss stink of evil itself, but exactly zero of the other Outer Planes kowtow to them, regardless of what the fiends there claim.
Some places - like Sigil - are more useful than others, though. Just because it's not the center of the universe, don't think it ain't important, berk.