The killer app for Apple's W1 chip is CarPlay. Assuming a modest cost to create the chip, Apple could offer the chip to automakers to embed in the dash at no-cost to permit one-tap pairing, a more stable and smarter connection, etc. Pairing headphones is painful but pairing in a car is worse as automotive systems often have two different Bluetooth profiles for the phone and audio – in particular, Siri runs over the phone profile and so will interrupt audio streams.
Headphones traditionally (but maybe not for long) use wires but cars traditionally use wireless. I think W1 is starting with headphones but is intended to be extended out to a range of IoT devices and headunits, including and especially the car.