Supporting tree-shaking of:

var p = {
  a: 'a',
  b: 'b'
};
externalFn(p.a);

into:;

var p = {
  a: 'a'
};
externalFn(p.a);

So basically, knowing that the object itself is not exposed and then being able to remove properties known not to be used at all.

Ideally to extend this into cases like:

var p = {};
p.a = 'a';
p.b = 'b';
externalFn(p.a);

as well, also supporting the same sort of thing.

The same principles can likely apply to class shape too, although there's much more exposure for classes potentially to track, likely with a more costly analysis and higher opt-out rate so worth weighing that up separately perhaps.