Halliday & Matthiessen (2004: 642-3, 644):
When a sequence is realised metaphorically by a clause, this means not only that it is mapped onto the transitivity patterns of the clause but also that it falls within the domain of Theme + Rheme organisation of the clause and also, by extension, that of the Given + New organisation of the information unit. … There is thus a gain in textual meaning in the shift from the congruent mode of realisation to the metaphoric mode.
… when a sequence is realised by a clause rather than by a clause nexus, it will be structured textually into Theme + Rheme and, since a clause is an information unit in the unmarked case, also into Given + New. This means that the figures that make up the sequence can be given thematic or newsworthy status.