I'm still trying to figure out how to make SplitView really useful. If you have email or Messages open on one side and respond, the keyboard mostly obliterates the other app.
I can see that big goofy keyboard eating up a lot of real estate. Indeed it does.
This SplitView thing is handy with a hardware keyboard since the software keyboard will be out-of-view. The shortcuts row will still take up a sliver on the bottom of the screen unless it's disabled in Settings/General/Keyboard/Shortcuts. (I wish shortcuts could be disabled automagically when a hardware keyboard is connected. Enhancement Request!)
A good use case for the side-by-side stuff yesterday I went through was setting up the device and needing to flip back to my password manager (1Password) frequently. Safari has an extension for 1Password and it's mighty convenient there; but most other apps do not. Traditionally one would have to double-tap Home and flip-flop between 1Password and whatever app needed that password. In some cases, depending in that app you're in, the password dialog would disappear or reinitialize or whatever when flip-flipping between it and 1Password. So you'd need to have that password pre-copied in the paste buffer before the time it's actually needed in whatever app you need it for.
I was setting up Family Sharing and needed iCloud credentials for both myself and my wife. If you've ever set this up you know it's done via the Settings/iCloud section, and it's a popup box that you progress through to add all the info. It was just infinitely easier & fluid to have both Settings and 1Password side-by-side rather than Home double-tapping.
There's definitely a lot of use cases, many of them involving chatting with other people in one app while looking up info or doing some kind of work in another. It needs some polish though; there's no reason to show *every* capable app in the slide-out app chooser, which are listed in a random order (not even alphabetical). Needs some organization there. No doubt Apple will address this kind of stuff down the road.