Halliday & Matthiessen (1999: 513):
The classes of word may be distinguished by their internal form, or by the way they are able to enter into larger constructions (or both). Typically the most complex is the class of circumstantial elements, because these are themselves often formed as complex constructions; there may be simple words (a class of adverbs), but there may also be constructs like English prepositional phrases, the function of which is to bring in other potential participants but to bring them in indirectly, like the sea in across the sea. The theory behind this is that there are two ways in which an entity can be involved: either directly as a participant in the process, or indirectly in a circumstantial role, such as the place where the process happens. This indirect participant is often construed as participating in a kind of secondary process tangential to the main one (grammatically, a prepositional phrase is a reduced variant of a clause).