Comment by BeetleB
Basic undergrad statistics. This doesn't make me better at doing statistics, but now I can understand things I read. Whereas prior to SR, I had learned the material three separate times - always forgot because of lack of use. SR made it stick.
Algorithms and data structures.
Basics of HTML/CSS/JS. I'm not a frontend developer, but this was enough for me to (mostly) understand colleagues' JS code. And often I would inform him of one of the newer JS features he didn't know of (e.g. null coalescing operator). Does it make me a JS developer? No. But it ensures I'm not useless at it.
Python 3.x new features. Simple things like "Stop using os.walk and use scandir instead".
A whole lot of Emacs keybindings. I was a heavy Emacs user before SR, but this really helped take it to the next level (I now mostly rely on hydras, so I no longer memorize keystrokes, cut I can't deny its effectiveness).
Some amount of elisp.
Probably a whole lot more random miscellany I can't recall right now.
Basically, what it does is let you retain information without usage. Prior to this, I would mostly retain only things I use (or had used) often.
I was in university for over a decade. Took lots of notes. But they're useless if you don't review them. Some years after leaving university I stopped trying to learn anything technical unless I was putting it to immediate use. Why bother if you're going to forget?
SR is what let me get back to studying for fun.
I can understand using SR for languages or I suppose geography or history trivia.
However, how do you use for skills/domains where you have to actively think?
Like in your Python example, knowing about os.scandir() would be a tiny bit helpful before Pathlib.
Let's say I put pathlib.Path().iterdir() in my Anki card, what would be the point of that for someone who is happily using glob or rglob?
What I mean is that for many domains it seems the challenge is what to put on these Anki cards.
Let's go with another example. Let's say you create Anki card Recall = TP / (TP + FN) for your statistics 101. However knowing why and when recall is important would be crucial instead of knowing bare formula.