Halliday & Matthiessen (2014: 731):
In the most general terms, the purpose of analysing a text is to explain the impact that it makes: why it means what it does, and why it gives the particular impression that it does. … What the metaphorical interpretation does is to suggest how an instance in the text may be referred to the system of the language as a whole. It is therefore an important link in the total chain of explanations whereby we relate the text to the system. A text is meaningful because it is an actualisation of the potential that constitutes the linguistic system; it is for this reason that the study of discourse (‘text linguistics’) cannot properly be separated from the study of the grammar that lies behind it.