Halliday (2008: 85):
… whichever of these rôles we are adopting, we need to observe from both ends. The grammarian, however system-oriented he may be, has to monitor instances of discourse; the discourse analyst, however text-oriented, has to keep an eye on the overall potential. The complementarity means that, unless you shift your angle, you will distort the picture: you cannot know all that is going on if you keep to just one observational perspective.