Halliday & Matthiessen (2014: 328-9):
Rounding off our discussion of circumstances, let us now review their status in the grammar, focusing on the prepositional phrases in the grammar since most circumstances are realised by prepositional phrases. Issues arise because while prepositional phrases serve as circumstances by default, they can also serve as participants in the clause, and even as elements of groups (nominal or adverbial). Issues also arrive because while prepositions serve in prepositional phrases by default, they can also come to serve as extensions of verbs, so-called phrasal verbs (and we can also note conjunctive prepositions, prepositions used as structural conjunctions in bound clauses, as in who will authorise payment on ascertaining that the item was really received.