Halliday & Matthiessen (2014: 109):
Why do these items favour thematic position in the clause – or, to put the question more meaningfully, why are they associated with thematic function, either characteristically or, in some cases [continuatives and conjunctions], inherently? In the most general sense, they are all natural Themes: if the speaker, or writer, is making explicit the way the clause relates to the surrounding discourse (textual), or projecting his/her own angle on the value of what the clause is saying (interpersonal), it is natural to set up such expressions as the point of departure. The message begins with ‘let me tell you how this fits in’, and/or ‘let me tell you what I think about this’.