Halliday & Matthiessen (1999: 531):
In conjunction, the various logical-semantic relations of expansion that construe clause-complex structures (discussed above under the "logical" metafunction) are deployed instead as a source of cohesion. There are a large number of such conjunctive expressions, ranging from single words like however, moreover, otherwise (many of them originally composite forms) to prepositional phrases like in that case, in other words, at the same time (often containing a reference word inside them). They cover more or less the same range of meanings that we referred to as "elaborating", "extending" and "enhancing"; but they do not establish any structural relationship in the grammar, and this is recognised in written English, where they regularly occur following a full stop.