Halliday & Matthiessen (1999: 24):
Of course, not every instance of the use of language involves the creation of new meanings. The greater part of most discourse consists of wordings which have been constructed on countless previous occasions — in the language, in the individual, and even in the course of the text. When we come across the sentence Rain is expected in the northern part of the region, falling as snow over high ground we recognise probably all of it as something that is ready to hand: not only has it occurred in the English language many times before, but the same writer has probably written it many times before, and many of these instances could be seen as forming part of the same discourse (that is, day-by-day weather reports in a sense constitute one continuous text). The storing of meanings for repetitive use and reuse is just as important as the potential for creating new ones.
The production of discourse by an individual speaker or writer can be seen as a dialectic between these two semiotic activities: between(i) recycling elements, figures and sequences that that individual has used many times before, and so for him or her are already fully codified, and
(ii) constructing new ones that are being codified for the first time (and some of which may remain codified for future use — especially with a child who is learning the system).