Full Idea
In modal logic the concepts of necessity and counterfactuals are not interdefinable, so the language needs two primitives to represent them, but with the machinery of possible worlds they are defined by what is the case in all worlds, or close worlds.
Clarification
'Primitives' are terms defined as basic, which cannot be analysed
Gist of Idea
Possible worlds make it possible to define necessity and counterfactuals without new primitives
Source
Joseph Melia (Modality [2003], Ch.1)
Book Reference
Melia,Joseph: 'Modality' [Acumen 2003], p.19
A Reaction
If your motivation is to reduce ontology to the barest of minimums (which it was for David Lewis) then it is paradoxical that the existence of possible worlds may be the way to achieve it. I doubt, though, whether a commitment to their reality is needed.