My SO recently bought an electric bike using this sort of scheme, not from Apple obviously, but the monthly payments toward the 3500$ bike were less than what it cost to commute to work using public transport before that. So it turned from an being a non trivial investment to an obvious thing to do.

The value proposition to me seems to be that they sold a bike at 3500$ they would not have sold otherwise and they can deal with the payment being made over time rather than all at purchase time.

For the user the value proposition is that value adding purchases like this can be made and paid off while they provide value.