What can Sketch do that Figma can't?

Work offline, Better font rendering

Better support for the built-in accessibility features of macOS.

I wonder how many people with visual accessibility issues would take a graphic design job

Accessibility features are used by people regardless of whether they have visual accessibility issues. For example, I (and many others–watch some WWDC presentations closely and you'll see this) use the "display zoom" feature during presentations and when doing UI work to focus on things. The feature where you wiggle the mouse cursor to make it big? That's an accessibility feature, but it got a prime spot at the keynote when it was announced. I have subtitles enabled all the time since it helps me catch things in videos if I miss what someone said. macOS has text-to-speech, which is quite useful to read me back things I write for grammatical errors or typos. Accessibility features can and are used by everyone. (Disclaimer: I have partial colorblindness and wear glasses.)

You can use voiceover as a way to add a lot of context and “color” to a UI. I make a big effort to support voiceover in my apps. In fact, most of the text that I translate is never seen by the user[0].

On the Mac, I also like to use tooltips, applying the same string as voiceover.

[0] https://github.com/RiftValleySoftware/ambiamara/blob/bbb1d98... (Note, also, that these voiceover strings have been localized in French, German, and Spanish).

of all the features you mention, except TTS none depend on the app being written in a specific framework as far as I know ? (and I don't remember TTS not working in e.g. Qt apps for instance - at least I know that I sometimes bash my keyboards repeatedly to stop the damn thing that I opened by whatever goddamn shortcut :D)

This was a direct response to your comment, so I was mainly focused on what accessibility features I use. However, one I missed but I use all the time is being able to control applications using accessibility APIs.

They could certainly take copywriting and proofing jobs.

Run natively on the Mac /s

Why /s though? It is a huge pro for some people.

Because it's already been mentioned a number of times in the thread and is a very major selling point (IMO).

Custom keyboard shortcuts