Moving Siri to the sleep/wake button adds yet another bit of functionality to a growing list of things the button already does. You press the button to lock or wake up your iPhone; you hold it down longer to power down the device.
And critically, the new Emergency SOS option in iOS 11 is also tied to the sleep/wake button, allowing you to automatically dial your country’s respective emergency number(e.g. 911 for the US 191 for Ghana) with five quick presses of the button. This same feature is linked to the “cop mode” setting — also new to iOS 11 — that temporarily disables Touch ID until the next time your passcode is entered.