Subjects> empty it

Whenever a pronoun refers back to an item that has already been introduced into the discourse, the pronoun is making anaphoric reference.

There are, however, contexts in which it does not have anaphoric reference.

It is hot!
It's foggy in Seattle. 

Except for the imperative construction, English sentences demand subjects and it is acceptably neutral in those cases where there is not a semantically meaningful subject.