Halliday & Matthiessen (2014: 110):
The conjunctive Adjuncts (often called ‘discourse Adjuncts’), as noted above, cover roughly the same semantic space as the conjunctions; but whereas conjunctions set up a grammatical (systemic-structural) relationship with another clause, which may be either preceding or following, the relationship established by conjunctive Adjuncts, while semantically cohesive, is not a structural one (hence they can relate only to what has gone before). These Adjuncts often are thematic; but they do not have to be. We may have either therefore the scheme was abandoned, with therefore as textual Theme, or the scheme was therefore abandoned, with therefore falling within the Rheme. Note how the Theme + Rheme analysis enables us to explain the difference in meaning between pairs of agnate clauses such as these.