Halliday & Matthiessen (2014: 109):
Those that are inherently thematic are the (textual) continuatives and conjunctions. As the language evolved, they have, as it were, migrated to the front of the clause and stayed there. Essentially they constitute a setting for the clause (continuative), or else they locate it in a specific logical-semantic relationship to another clause in the neighbourhood (conjunction). In either case, their thematic status comes as part of a package, along with their particular discursive force.