Halliday & Matthiessen (2014: 193):
In general, every free clause in English requires a Subject, because without a Subject it is impossible to express the mood of the clause, at least in the usual fashion. We have already noted that the difference between declarative and yes/no interrogative is realised by the order of the elements Subject and Finite; and it is impossible to arrange two elements in order if one of them is not there. So while the it in it’s raining, and the there in there was a crash, do not represent any entity participating in the process of raining or of crashing, they are needed in order to distinguish these from is it raining, was there a crash.