Halliday & Matthiessen (2014: 329):
At the same time, there are many instances where a nominal group seems to have more or less the same function whether it is brought into the clause directly, or indirectly via a prepositional phrase: for example, John in sent John a message/sent a message to John. We have interpreted these as participant functions, rather than as circumstantial elements… . But they also suggest that the line between participants and circumstances is not a very clear one, and that the preposition does function like some highly generalised kind of process, by reference to which the nominal group that is attached to it establishes a participant status.