It's not an accessory. It will function completely on it's own for the reason which it was made. I know I have to tell you because 95% of people still don't get it. It's being made for biometrics. Will it tell you of notifications from your iPhone? Yes of course.... That's absolutely not why it's being made.
Biosensors are the function of this device. That's what it's for.
Of course, I know that its main thing will be the biosensors but I still find it hard to believe it'll be able to process all the data collected from the sensors and maybe have internet (even wifi would be impressive) on its own. Simple rationale I have in my mind just tells me that it has to use another devices (iPhone, iPad) resources via Bluetooth in order to achieve desired size for design and functionality purposes.