Halliday & Matthiessen (2004: 295-6):
… the choice of ‘plus or minus preposition’ with Agent, Beneficiary and Range … serves a textual function. … The principle is as follows. If a participant other than the Medium is in a place of prominence in the message, it tends to take a preposition (ie to be construed as ‘indirect’ participant); otherwise it does not. Prominence in the message means functioning either
(i) as marked Theme (ie Theme but not Subject) or
(ii) as ‘late news’ — that is, occurring after some other participant, or circumstance, that already follows the Process.
In other words, prominence comes from occurring either earlier or later than expected in the clause; and it is this that is being reinforced by the presence of the preposition. The preposition has become a signal of special status in the message.