I do not know why anyone would need it close an iPhone (power off), without having to press the button sleep / power, but it is a feature of iOS 11 that allows this.
In addition to the many new features brought in iOS 11, Apple added to the menu "Settings” and an option through which you can close your iPhone, iPad or iPod touch, without having to hold down the "" button for a few secondssleep" / "power", until it appears "slide to power off".
The new option, called "Shut Down"Was added next to the reset option, in"Settings”→“ General ”.
It is not known exactly what this option helps at the moment, but it is very possible that Apple to prepare new functions for the current "Power" / " buttonSleep".
Most likely, this new option helps us to close the iPhone, iPad or iPod in the case of one mechanical defects of the button sleep. The restart will be done simply power cable connection lightning to the device.