Halliday & Matthiessen (2014: 110):
If there is a Vocative in the clause, or a modal or comment Adjunct, it is quite likely to be thematic: these items are characteristic of dialogue, in which the speaker may be calling the attention of the listener, or else expressing his or her own angle on the matter in hand, whether probable, desirable and so on, and hence they tend to be brought in as key signature to the particular move in the exchange – in other words, as Theme of the clause.