What must you do to summon a spirit, or whatever? No, take the question one step further back. Where do spirits and their ilk hang when they're not being summoned? Somewhere else, obviously. On the astral plane, maybe, or on one of the "metaplanes" (whatever the frag they are ...). Bringing them across takes effort. It takes magical jam, and—from what I've heard—to drag the big boys, kicking and screaming, into the material world, it can really harsh a spellworm out.