It needs to justify its existence no more than any other watch — mechanical or electronic — ever made. Of course you don’t need it. No one, not one person on the face of the earth, needs any $400 watch, Apple Watch or otherwise.
The right question is simply “Do you want one?”
It’s about desire, not necessity. Convenience, fun, and style are not needs. They’re wants. And people will gladly pay for what they want. The iPad faced similar misguided criticism. How many times did you hear or read someone say of the iPad, “Why would anyone who already has a phone and a laptop need an iPad?” That was the wrong question, because almost no one needed an iPad. The right question was “Why would someone who has a phone and laptop also want an iPad?”
The emotional and style aspects of the Apple Watch are the things many tech nerds and engineers just don’t get.