Halliday & Matthiessen (1999: 316):
As we have shown in the context of grammatical metaphor, the choice among alternative construals is made on the basis of both ideational and textual factors. These factors ‘conspire’ together so that different strategies are favoured in different registers: the congruent form (sequences) in casual speech, the metaphoric form (figures of being & having) in elaborated forms of writing.