It [WH- element] has become part of the system of the language, and the explanation for this lies in the thematic significance that is attached to first position in the English clause. Interrogatives express questions; the natural theme of a question is ʻI want to be told somethingʼ; the answer required is either a piece of information or an indication of polarity. So the realization of interrogative mood involves selecting an element that indicates the kind of answer required, and putting it at the beginning of the clause.
- Halliday (1985), Pag. 48