Desktop shines when you create something novel.
It's as simple as that -- there are tons of things people have created really polished iOS "happy path" software to create on, but they're not general purpose creators.
Specifically because anything general purpose threatens Apple's App Store toll gate, and thus cannot be allowed to exist.
If apple would just get over themselves and allow sideloading (or, gosh, a terminal!), it would go a long way to improving iOS usability for creatives.
…as long as it’s sanctioned by the platform owners.
The main one for me is the longevity of your data. It is connected to the control over your data you have in a desktop OS. Good luck finding what you did five years ago in an app in some device and reproducing it now. You are at the mercy of some proprietary format the app creator chose, the availability of the app at a later stage, whether you saved your work in a cloud service, whether you kept your old device running, the operating system updates you installed, .....
With a desktop you can archive your work, whatever it might be, in sequences of bits that reside in your disk. Open source software and virtual machines go a long way for helping reproducibility.
Not all tasks benefit from reproducibility and longevity but I'd argue they include a lot more than computer programming
For most creation-oriented tasks I do that aren't computer-centric, I'd much rather have an iPhone or iPad than a laptop. They're great tools for creation & work, just not so much for creating stuff for computers, certain exceptions aside, and for those they're mostly just about as good as a laptop, or even not-very-good but usable in a pinch, not better.