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Full Idea
In S5, necessity and possibility are not themselves contingent matters. This is plausible for metaphysical modality, since metaphysical possibility, unlike practical possibility, does not depend on the contingencies of one's situation.
Gist of Idea
If metaphysical possibility is not a contingent matter, then S5 seems to suit it best
Source
Timothy Williamson (Truthmakers and Converse Barcan Formula [1999], §1)
Book Ref
-: 'Dialectica' [-], p.255
A Reaction
This is the clearest statement I have found of why S5 might be preferable for metaphysics. See Nathan Salmon for the rival view. Williamson's point sounds pretty persuasive to me.