Vague hypothesis: The Apple Watch is a test of Apple's ability to break the link between usefulness and price. "Smartwatch" is, vaguely, a gadget category, so Apple can sell it to the people who buy its gadgets. But "$10,000 smartwatch" is not, and Apple can also see whether people will spend $10,000 on a stupid luxury product from Apple. And if they do, why not a $20,000 gold-plated iPhone or whatever? The Apple Pocket Watch, advertise it in Vogue.
[Update: I typed that ello like 30 seconds before this.]