Halliday & Matthiessen (2014: 237):
The two functions of Recipient and Client resemble one another in that both construe a benefactive rôle; they represent a participant that is benefiting from the performance of the process. The Recipient is the one that goods are given to; the Client is one that services are done for. Either may appear with or without a preposition, depending on its position in the clause … the preposition is to with Recipient, for with Client.