Halliday & Matthiessen (2014: 320-1, 322):
The circumstantial element of Cause construes the reason why the process is actualised. It includes not only Reason in the narrow sense of existing conditions leading to the actualisation of the process, but also Purpose in the sense of intended conditions for which the process is actualised (what has been called ‘final cause’). Both Reason and Purpose tend to be eventive (and are therefore commonly construed as clauses in a clause nexus); but there is another type of Cause that tends to denote a person — the circumstance of Behalf. …
The semantic relations of reason and purpose tend to be realised as separate clauses rather than as phrases within the clause;