Halliday & Matthiessen (2014: 457):
The clause complex is of particular interest in spoken language, because it represents the dynamic potential of the system — the ability to ‘choreograph’ very long and intricate patterns of semantic movement while maintaining a continuous flow of discourse that is coherent without being constructional. This kind of flow is very uncharacteristic of written language. Since grammatical theory evolved as the study of written language, it is good at synoptic-type ‘product’ representations, with constituency as the organising concept, but bad at dynamic-type ‘process’ representations, which is what is needed for the interpretation of speech.