Full Idea
It has proved difficult to justify possible worlds semantics without accepting possible worlds. Without a secure metaphysical underpinning, the results in logic are in danger of having nothing more than a formal significance.
Clarification
'Semantics' concerns meaning
Gist of Idea
If possible worlds semantics is not realist about possible worlds, logic becomes merely formal
Source
Joseph Melia (Modality [2003], Ch.2)
Book Reference
Melia,Joseph: 'Modality' [Acumen 2003], p.62
A Reaction
This makes nicely clear why Lewis's controversial modal realism has to be taken seriously. It appears that the key problem is truth, because that is needed to define validity, but you can't have truth without some sort of metaphysics.