Halliday & Matthiessen (2004: 83-4):
… 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.