Halliday & Matthiessen (2014: 199):
In Theme predication, the final clause is a relative clause functioning as Post-modifier to the it (where it means ‘the thing that’, ‘the time that/when’ and so on). The clause as postposed Subject, on the other hand, is a fact clause … and it is related to the it by apposition (paratactic elaboration).
… a clause with predicated Theme always has the verb be, and has a non-predicated agnate … A clause with postposed Subject has no such agnate form; moreover such clauses are not restricted to the verb be. Being facts they typically occur in clauses where the proposition has an interpersonal loading; for example, a Complement expressing modality or comment (it is possible/unfortunate that …), or a Predicator expressing affection or cognition (it worries/puzzles me that …).