Halliday & Matthiessen (2014: 329):
Most circumstances are realised by prepositional phrases. A prepositional phrase can be interpreted as a shrunken clause, in which the preposition serves as a ‘minor process’, interpreted as a kind of mini-verb, and the nominal group as a participant in this minor process. … The preposition, it was suggested, acts as a kind of intermediary whereby a nominal element can be introduced as an ‘indirect’ participant in the main process.