Halliday & Matthiessen (2014: 704, 705):
As we have noted, metaphors of mood make it possible for the semantic system of SPEECH FUNCTION to be further elaborated in delicacy. Why does the speech functional system need to be expanded in this way? The basic principle is this: the expansion of the speech functional system has increased the meaning potential available to interactants for negotiation in dialogue. …
The potential for negotiation in dialogue created by metaphors of mood is directly related to the different contextual variables within tenor. Metaphors of mood expand the interpersonal resources for negotiation, whether the negotiation involves consensus or conflict. The tenor variables are usually discussed in terms of status, formality, face, tact and politeness (and their negative counterparts). What these have in common is a very general sense of the social distance between the speaker and the addressee. Here interpersonal metaphor is part of a principle of interpersonal iconicity: metaphorical variants create a greater semiotic distance between meaning and wording, and this enacts a greater social distance between speaker and addressee. The semiotic distance is often manifested directly in the lexicogrammar as a syntagmatic extension of the wording.