Halliday & Matthiessen (1999: 399-400):
What is significant here [in theme predication] is that the textual organisation is realised by the second-order resource of grammatical metaphor. That is, the grammar is as it were turned back on itself: it reconstrues itself with a particular effect in the discourse, as diagrammed in Figure 9-15.
We can see, then that experiential grammatical metaphor is a strategy for creating a 'carrier' of textual meanings.