Why has the 777x been designed with folding wingtips? I recently visited the Boeing factory outside Seattle, WA and noted in the distance a new 777x being built with, what looked like very large winglets.

However, on closer inspection they're not - the ends of the wings are designed to fold up.


Source: aviation today

I didn't get a chance to ask our tour guide, so I was wondering if anyone here can explain why this aircraft has been designed with folding wingtips. I'm presuming this is only for ground operations, and my guess is something to do with size - but are they really so big that they need to fold up? I would have imagined that other large wide body jetliners would have similar dimensions.

Please note that I'm not asking about whether this should have winglets, or if winglets provide any benefits, I'm asking specifically why the ends of these wings fold up rather than just fixed as per previous 777 models.
The Boeing 777X website states that this is to enable a more efficient wing (read: wider span) while maintaining the airport gate and taxi footprint of the classic 777 (which ensures airlines can use the 777X on roughly the same airports and intermix operations with the classic fleet).