Had to google it.

"It's because of the fear that the home button may be broken. iPhones are not cheap in China so people take care of them while using. Several years ago people began to complain about their home button being easily broken and it has somehow been a widely recognized truth, so even the home buttons are not that easy to be broken, they tend to use AssistiveTouch instead. When you buy an iPhone in China the salesman would automatically turn on this function while helping you to do the settings. I myself have not experienced a broken home button during 4 years with my iPhone 4; however, I did have a broken sleep button from my 3rd year, which proved that the rumours are, in some way, true. So I turned on AssistiveTouch..."

source: http://uk.businessinsider.com/chinese-use-assistivetouch-on-...

I've had to do that to almost every iPhone I've used except the previous 6 I had.

I'm not sure why I went through them quickly. I started to use the software home button right away on the 6 and it's still kickin. The 6 Plus I didn't do it to misses a click every so often.