Paul Corfield wrote:

Sorry but this time you are incorrect. The original post referred to a
leasing arrangement for the class 378 *electric* stock and not the
diesel order.

As TfL made clear at the time, the reason for using the traditional RoSCo
method of acquiring stock for the Tottenham and Hampstead was so that they
wouldn't be left with a fleet of useless trains if/when the line is
eventually electrified. They could return them to the RoSCo for use
elsewhere and then acquire new electric units directly, as per the 378s.