Namely the choice to get a mechanical replacement mechanism after the warranty expires. Sure, it's not going full Monty but at the same time it gives you a chance to enjoy the watch long after your Iwatch has been consigned to the dustbin (and not of history). Let's face it, smart watches, just like phones, are a disposable product with a half life of a year and a half, if that.
So the cost per wear or period of time is likely to be much higher than with a much more expensive mechanical watch, simply because of the short lifespan.
Offering a second life, so to say, is therefore quite an inspired choice IMHO.