While it currently allows 24 monthly re-payments at zero interest, this hints at a rental (you never own it) with recurrent automatic upgrades. Apple does not even have to reveal purchase or trade-in prices at all. It keeps you in the tent where Apple can earn more money from its services – forever.
And the benefits for Apple are enormous. No finance costs leaking to third parties, and it is really only holding the manufactured value (circa A$200), not the retail value of the iPhone on its books. Very clever.
On the plus side, Apple and Samsung are the only two smartphone makers offering trade-in programs, so these are the safest purchases.