Halliday & Matthiessen (1999: 172-3):
They are really reduced or minor figures, functioning as elements inside other figures. The preposition is a kind of mini-verb; the line between circumstances and figures is a very fuzzy one, and we often find agnate expressions where one is a prepositional phrase and the other is a non-finite clause: cf. I washed it using sugar soap, she came accompanied by her children. The entity that occurs inside the macro circumstance is therefore already entering into a relationship with a reduced form of a process; its participation in the main process is thus mediated and oblique. We can thus contrast the different statuses of two entities where one is a direct participant and the other enters in circumstantially; e.g. this dictionary was published in two volumes, where this dictionary is Goal while two volumes enters into the publishing process indirectly in a circumstance of Rôle.