Not everyone is you. Not everyone wears an Apple Watch. Not everyone even has an iPhone. If you do, great- there may not be a use case for you. But again, not everyone is you.
This product- if it comes to market- may be aimed at the
billions of other people that don't have Apple Watches & iPhones. I know that can be almost impossibly difficult to imagine but one can easily do the math to know that it is true.
Or it may be aimed at families in which only one or some have an iPhone that often leaves that house with that one or some. When a watch and/or phone leaves a home, those left behind might still want to use Siri to do something as mundane as play some music... rather than waiting until a watch or phone comes home.
Or maybe it's entirely useless but Apple sees competitor products selling in enough volume to roll one out just as a pure play for the revenue. Why do we need Beats headphones? Why do we need bluetooth earbuds? Why do we need another watch? Why do we need an iPhone? Depending on an individuals own situation, why do we need anything from Apple? Maybe we do... or don't... but for those who are not us... maybe they don't... or do.
Apparently, Apple thinks a slice of a market wants one of these from Apple. I suspect Apple could put poop in a box and brand it iPoop or Apple Poop and sell huge numbers of boxes with some proclaiming it as "the finest poop ever" and "how did we ever get by without this poop?" In other words, Apple seems to be in a position to box up just about anything and sell a bunch of that anything on brand momentum alone. This particular thing doesn't seem to be a big stray for Apple's current offerings. So why not?
If everyone is you, nobody will buy and Apple will learn their lesson (to check with you first).