The flat floor in the rear of the cabin provides additional legroom for rear passengers, but it's even more useful than that.
I would often chuck my grocery shop or ungainly objects back there, especially if I already had something in the boot, or if I needed them to stay upright.
I also often used the rear seat myself when I was on-the-go. I attended multiple online events in this car, and its ability to function as a mobile office, with ample space in the back seat and power outlet for charging my laptop plus USB-C for charging my phone while it tethered the internet, was invaluable at times.
I would also use it this way while waiting for the car to charge up and I had deadlines to meet. Electric future, indeed.
The rear doors are also large and open very wide, so if you had kids I imagine it would also be handy for fitting child seats or the like.