I don't personally use this method, but it seems to me like this would help with all the cases where you search for something on stackoverflow, use the solution, and 3+ months later you encounter the exact same issue. You remember that you already had this issue, and you remember that you searched on SO and found a solution, but you don't remember what it was. If you wrote a flash-card and learned it, you'd have that knowledge available immediately. Or, let's say you're learning a new language/framework that you want to use professionally, but don't use frequently yet. So you start a project in that language/framework, learn some of the things that make it different, and every time something surprised you or you had to search for a solution, you write a flashcard. Then you can learn these aspects separately from having to work on a project, and once you do have a use for it, you have the knowledge available.

I think this could be less likely if you're very specialized, but if you're a consultant or freelancer and have to work with lots of different technologies, locking down the differences and unique aspects for each could save a lot of time.