Halliday & Matthiessen (2004: 393):
Here the dependent clause is given thematic status. … In general, thematic beta-clauses serve to set up a local context in the discourse for the alpha-clause: they re-orient the development (as in the staging of a narrative), often distilling some aspect of what has gone before to provide a point of departure for the dominant clause, thus creating a link to the previous discourse. … The textual domain of a thematic dependent clause is often a sub-complex rather than just a single clause, and it may even extend beyond the clause complex in which the clause serves.