Halliday & Matthiessen (2014: 699-700):
This is the general effect of grammatical metaphor: it construes additional layers of meaning and wording. To capture this layering in our grammatical analysis, we introduce one or more additional structural layers in the box diagram. The representation of grammatical metaphor in such diagrams shows how the metaphor is embodied in the structural organisation as an increase in the layers of meaning and wording.
But there is, of course, also a systemic effect. Systemically, metaphor leads to an expansion of the meaning potential: by creating new patterns of structural realisation, it opens up new systemic domains of meaning. And it is the pressure to expand the meaning potential that in fact lies behind the development of metaphorical modes of meaning. Thus in the system of MODALITY, the system of ORIENTATION is expanded by the addition of a systemic contrast in MANIFESTATION between ‘explicit’ and ‘implicit’: the metaphorical modalities described above make it possible to make the orientation explicit in wordings such as I think and it is likely that, which, in turn, makes it possible to increase the delicacy of differentiations (cf. I think/imagine/expect/assume/suppose/reckon/guess; I would think/I would have thought; I imagine/I can imagine; and so on).
As we have already seen, this same principle extends beyond modality and applies to modal assessment more generally (e.g. I regret, it is regrettable that). The metaphoric strategy is to upgrade the interpersonal assessment from group rank to clause rank – from an adverbial group or prepositional phrase serving within a simple clause to a clause serving within a clause nexus of projection.