Halliday & Matthiessen (2014: 434):
In the creation of text, we choose between augmenting a clause ‘internally’ by means of a circumstantial element and augmenting it ‘externally’ by means of another clause in a complex. The decision depends on many factors; but the basic consideration has to do with how much textual, interpersonal and experiential semiotic ‘weight’ is to be assigned to the unit: the more weight it has, the more likely it is to be constructed as an interdependent clause in a clause complex rather than as a circumstantial phrase (or adverbial group) augmenting a clause.