I don't think you understand, it's not a feature it's a fail safe if the actual home button fails. Here's one example of why this is not a feature: You won't be able to just hit the home button when the device is in standby to light up the screen you will need to hit the power button and then enter a passcode.
Edit: from google, how to enable onscreen home button in iOS 9
This is how you do it:
- Open Settings.
- Go to General > Accessibility.
- Scroll down to the section labeled INTERACTION and tap on AssistiveTouch.
- On the next screen, toggle AssistiveTouch to the green On position.
- A white circle with a grey box will appear on screen. Tap this circle to expand it to a big box on screen.