If you have ever noticed your iPhone's screen randomly drop down halfway you might have assumed that there was some sort of a bug error and that pushing the home button will get rid of it, or it was a feature that you were unaware of at first. But the iPhone's screen dropping down after you double-tap the home button is actually a very important feature and that of an accessibility.
Back in the day when Steve Jobs was still CEO of Apple, he insisted that the iPhone of the time be easy to use and a bigger screen size would compromise that. However, in 2012, the iPhone's competition from Samsung was stronger than ever as they offered large screen sizes and the data clearly showed that people wanted bigger screen sizes. Tim Cook wanted to offer people what they wanted, thus a few years later the biggest iPhones were released, that of the iPhone 6. However, to stay true to Steve Jobs' vision, the screen drop was an accessibility feature to allow a user to still operate a larger iPhone with one hand and reach all functions.
Take a look at the video below by the YouTube channel: Apple Explained on Why The iPhone's Screen Drop Down?