Apple finally announced new MacBook Pro's, and everyone is talking about the new TouchBar. Apple has confirmed that the new chip, called the T1, that powers the TouchBar runs a variant of watchOS. Everyone is saying how this computer inside a computer approach provides better hardware security, by allowing the T1 to manage Touch ID and access to the camera. I'm sure that's all true.
However, I don't believe security is the only reason for a separate chip and OS running inside the MacBook Pro. If Apple ever wants to make the TouchBar available on an iMac, which I assume they do, it will have to be on an external device. The new T1 chip will live in that external device so it can drive the TouchBar display and manage the Touch ID functions. I expect it will also have to handle the cryptography necessary to securely transmit touch ID events over USB to the iMac.
My guess is that Apple designed the T1 for the external device, and then figured out how to package it inside of the MacBook Pro so they could use the same software architecture, and largely the same hardware architecture, to power this new functionality.