Halliday & Matthiessen (2014: 709):
The interpersonal metafunction defines the environment in which children first learn the strategy of grammatical metaphor. No doubt this is partly because interpersonal metaphors tend to make selections more explicit, as when probability is realised by a ‘mental’ clause projecting the modalised proposition (‘explicit’ orientation), and partly because the interpretation of interpersonal metaphors is often both supported and ‘tested’ immediately in the ongoing dialogic interaction. For example:
Oh. Stefan, can you turn off the tape? – [Non-verbal response: tape is turned off.]
Here the responses show that the ‘yes/no interrogative’ clauses are interpreted as metaphorically realised commands rather than as congruently realised questions. The expansion of the interpersonal semantic system through grammatical metaphor provides speakers with additional, powerful resources for enacting social roles and relations in the complex network of relations that make up the fabric of a community of any kind.