There are so many random features like the sleep tracker (https://www.supermemo.com/help/sleep.htm) that don't really make much sense to have but are there anyway.
There's also a web version if you just want flashcards that's mainly focused on language learning. They sell some courses and you can add your own cards too.
The biggest selling point for me is the incremental reading (https://www.supermemo.com/help/read.htm) feature. It's the only way I can manage to keep track of so many articles that I find interesting, and is vastly superior to leaving hundreds of tabs open or a bunch of bookmarks I'll never look at. It's superior to instapaper,readitlater etc because I don't have to decide what to read when I open the app, and I don't have to worry about forgetting about the article if I stop in the middle of it.
If I see anything remotely interesting, I just chuck it in Supermemo, and I'll see it eventually. If it's really important, I'll set its priority higher so I see the article sooner.
If I'm doing research on something and have 15 tabs open, all 15 tabs go into Supermemo, and will eventually get whittled down into flashcards. Start with an article -> extracts -> smaller extracts -> cloze deleted flashcard.
Long comment chains on Hacker News? Supermemo. Youtube videos? Supermemo. Email? Supermemo. Images? Supermemo.
I check my RSS feeds once or twice a week, grab all the interesting articles, import into supermemo, and I'm done.
Supermemo for me replaces: Evernote/OneNote, most bookmarks, Anki, and various todo list apps.
If I just wanted to make flashcards from set material, I'd honestly use Anki, because of the mobile apps. It's really easy to do a few repetitions when you have downtime through the day.
Every person I've shown this app too doesn't really care about it, and I don't really know why. If I had to guess, it's probably because the UI is horrific. I can effortlessly keep up with so much information, and it takes an hour or two out of my day.