Managing files without having to go through all sorts of unnecessary contortions.
Running Final Cut Pro and similar pro editing tools. Being able to boot into Windows in order to play any of the many games that are neither available for iOS nor for MacOS. Neither of these (pro editing apps, Boot Camp/virtualization software) are currently my cuppa but if they matter to you, it could be a big deal.
I also find selecting, copying, and pasting text to be SO much easier with a mouse than with a finger. Sure you can now use a mouse or trackpad with the iPad Pro but I find the gigantic thumbprint of a cursor lacking in accuracy of focus and even the Magic Keyboard isn't nearly as responsive for these fine motor tasks as a mouse or trackpad is on a Mac.