Halliday & Matthiessen (2014: 143):
The general principle behind the expression of MOOD in the clause is as follows. The grammatical category that is characteristically used to exchange information is the indicative; within the category of indicative, the characteristic expression of a statement is the declarative, that of a question is the interrogative; and within the category of interrogative, there is a further distinction between yes-no interrogative, for polar questions, and WH- interrogative, for content questions.