Halliday (2008: 48-9):
So the lexicogrammar adopts two contrasting perspectives for construing all this complexity. The one is specific and open-ended; hence flexible, but low in information: this the lexical perspective, good for seeing phenomena as particular. The other is general and systemic: hence high in information, but creating closure: this is the grammatical perspective, good for seeing phenomena as generality. The two perspectives are complementary; any phenomenon can be looked at in terms of either, but they will present two different images of the whole.