Full Idea
The apparatus of possible worlds affords greater expressive power than mere talk of possibility and necessity. In particular, possible worlds talk allows us to introduce degrees of possibility.
Gist of Idea
Possible worlds allow us to talk about degrees of possibility
Source
Barbara Vetter (Essence and Potentiality [2010], §3)
A Reaction
A nice feature, but I'm not sure that either the proportion of possible worlds or the closeness of possible worlds captures what we actually mean by a certain degree of possibility. There is 'accidental closeness', or absence of contingency. See Vetter.