New and old GUI for the same functionality exist side-by-side in Win10

I am perplexed by the fact that the very familiar "Add new user" GUI and a new reduced functionality version seem to be existing side-by-side in Windows 10. Same with many other functions. Why? What is the purpose? Particularly since the old options both have more functionality and are easier to find. It's as if there are two separate GUIs - the new look white/grey and the older functionality that hasn't changed since XP or WIndows 7. Or is it just two different ways to access the same functionality - using different interfaces? What is the thinking and rationale for this at Microsoft?

Because the transition to the new GUI (Windows 8) was too abrupt and jarring for many users. Microsoft probably backed off and decided to ease the transition with Windows 10 and the inclusion of both new and old GUI. Windows 10 will phase out the old GUI.

Thanks! I prefer the old style because it's got more functionality and isn't as "dumbed down".
Windows tends to just use new wallpaper in the same rooms, so to speak. Under the hood a lot of the stuff I learnt as a teche on NT4 back in 1998 is still there, under the hood. The structure is the same etc. I hope they leave it as long as possible.
If they phase it out it'll be the nail in the coffin for me, together with the privacy concerns.

The dual nature is to accommodate both the traditional mouse pointer and keyboard method of desktop operation, with small areas of the screen available for drag and click operation, and the touchscreen of tablets and phone devices for tap, gesture, and swipe control.