Halliday & Matthiessen (2014: 126):
The significance of these [thematic] patterns emerges when we come to consider the importance of clause theme in the overall development of a text. By itself the choice of Theme in each particular instance, clause by clause, may seem a fairly haphazard matter; but it is not. The choice of clause Themes plays a fundamental part in the way discourse is organised; it is this, in fact, that constitutes what has been called the ‘method of development’ of the text (see e.g. Fries, 1981, and contributions to Ghadessy, 1995; and to Hasan & Fries, 1995). In this process, the main contribution comes from the thematic structure of independent clauses. But other clauses also come into the picture, and need to be taken account of in Theme–Rheme analysis.