A large chunk of learning any language is memorization - a lot of memorization. Children do it unconciously without any effort or special tools, but they still effectively memorize thousands of words to become fluent in their native language. After a certain age, most people's ability to easily learn thousands of words in a new language just by exposure gets heavily nerfed, but if they want to learn a new language, they will need to somehow be able to understand and produce thousands upon thousands of words in the new language. The best tool I know of to accomplish this is SRS. When I was living in Denmark (I was 16 at the time), I learned and maintained a sizeable inventory of words over the course of a year using SRS (I got many compliments about my vocabulary towards the end of that time), and it was a massive boon to being able to use the language effectively. It was not a substitute for actually practicing it (or other methods of practicing other aspects), but a very useful complement to real-life conversation.
I also have found Anki to be useful for learning concepts in mathematics and physics - again, it doesn't serve as the entirety of studying, but a very valuable complement, alongside reading technical expositions of the concepts and solving problems that make use of the concepts that I learn and reinforce using SRS.
Other things I use Anki for include memorizing poetry, remembering different musical riffs and motifs, practicing problems for chess and go, maintaining various English words (such as retrograde and prograde, leeward and windward, "junoesque", remembering when dawn and dusk are [I used to get these mixed up often], or how to spell "hors d'oeuvres"), various facts (such as that the moon is waxing when I can see it in the evening, and waning when it is visible in the morning, or the composition of the British House of Lords, or the basic idea behind how solar panels work), the flags of different countries, as well as the coordinates of various cities around the world, and various other things that I might forget, but want to remember.