Halliday & Matthiessen (1999: 532):
As far as the textual metafunction is concerned, therefore, any one clause will typically embody two sets of semantic choices. One will be its organisation as a message, a piece of information flowing from speaker to hearer, its limits defined by the speaker's point of departure and the focus of attention projected by the speaker on to the listener. The other will be the cohesion it sets up with the preceding moments of the discourse, as well as with other discourses and with the total semiotic environment. These enable the clause to function effectively as reflection and as action.
But in the course of serving this enabling role, the textual component opens up a new dimension of meaning potential, in that it construes a further plane of "reality" that is as it were made of language — meaning not as action or as reflection but as information. In the modern world, when we increasingly live by exchanging information, rather than by exchanging goods-&-services as hitherto, this aspect of meaning potential is coming more and more to be foregrounded But it has always been there; and this is not the first time in history that it has proved to be an indispensable resource.