The Short Version
WatchOS 27 brings a more capable Siri to Apple Watch, with doubled sensor components that could improve accuracy and battery life.
WatchOS 27's upgraded Siri turns Apple Watch into the device that answers you when your phone is buried in a bag, which is most of the time. Doubling the sensor components could also let the watch read your body more directly instead of leaning on software to fill in the blanks, and that's where accuracy and battery life both stand to improve. Apple's essentially asking the Watch to carry the AI experience on its own.