1. Memorizing Japanese vocabulary, pitch-accent, and basic grammar rules. It would probably be useful, at least to some degree, for any language like this.
2. Any certification which required strict memorization. All the basic ComptTIA certs were like this, and the CCNA:R&S cert (unfortunately) required memorizing commands and their syntax.
3. Verses in the Bible (though this is pretty basic in comparison, just Address <-> Text).
In all honestly I think one of the best things you can do if you need to memorize something is make the flash cards yourself, whether with Anki, another app, or even just index cards. This forces you to think about what it is that you're trying to memorize and phrase the text of the cards in a way that you understand. After that, reviewing is just kind of "maintenance" in my opinion. There was definitely a marked difference in my retention when I was using pre-made Anki decks vs. creating them myself. It is also easier to create cards using the desktop app.
It's a side note, but I also don't agree with Anki's pricing model. The app is free on Android[1] but $25 on iOS[2]. I think I heard (I don't have a source) that the developer's justification was that they needed to make money from the all the time and effort they spent creating Anki, plus hosting costs, etc - so why not do a cheaper price on both Android/iOS, or do a free-to-download app with a subscription model? For what it's worth you can use the web version on iOS but the app is a better experience IMO.
[1] https://play.google.com/store/apps/details?id=com.ichi2.anki
[2] https://apps.apple.com/us/app/ankimobile-flashcards/id373493...