With the iPhone 6 Plus and the new 6s Plus, you have the ability to use the home screen in landscape mode (with the iPhone held horizontally) or in portrait mode (the iPhone held vertically).

Given the preponderance of vertically-shot videos, you may not want the home screen to switch when your iPhone is in a horizontal position. Sure, it’ll move to a vertical position when you move your phone, but maybe you have your reasons for wanting the home screen to not switch at all. (Because annoying).
…
Read full post here:
https://www.techadvisor.co.uk/how-to/software/how-to-turn-off-or-turn-on-landscape-mode-on-the-iphone-6-plus-and-6s-plus-home-screen-3626367/