It’s not a necessity. People are buying it almost solely because Apply made this. If some generic technology company had made this, it likely would only sell a few million units at best, possibly not even a million.
And that is what has been observed. Apple did not really do anything new here.
“People are buying it almost solely because Apple made this.”
Not just because _Apple_ made it, but because some company created a “walled garden” ecosystem good enough that many millions of people have bought into, at significant personal cost, and that ecosystem means anything new added to their collection seamlessly integrates with & complements the other investments. That’s an achievement - and sales pitch - few have pulled off; the Android ecosystem is heading that way with their own “wearables” (https://android.com/wear/) but there’s so much fragmentation no particular “watch” is gaining significant traction (in close parallel, Android phones make almost no profit). Contrast that with the iPhone which, as a single product with few variations, dominates half the smartphone market - and the Watch single-handedly complements that “half the market”.
Insofar as “people are buying it almost solely because Apple made this”, well yeah: half the smartphone market is iPhones, and this complements the iPhone. Nobody else has such a huge market share with such a clear complement to their device.