Halliday & Matthiessen (2014: 310):
This last type [construed as it + a verb in the ‘present in present’ tense] is unique in English, in that it has no participant in it. The it serves the interpersonal function of Subject, like the there in an ‘existential’ clause, but has no function in transitivity — if you are told that it’s raining, you cannot ask What is? and the it cannot be theme–predicated (we cannot say it’s it that’s raining) or serve as an identified Theme or Rheme (we cannot say it is what’s raining/what’s raining is it). On the other hand the tense is clearly that of a ‘material’ process. These clauses can be analysed as consisting of a single element, the Process; they are the limiting case of a ‘material’ process clause.