It's mostly the convenience of always having the ability to create a spaced-repetition flashcard there in your browser.
Our primary use cases are studying for exams and language learning. Hopefully that sheds some more light on how the Chrome extension / Notion integration are used.
Of course it's hard to know whether other alternatives could have worked even better, but this way I devote 30 minutes a day which can be on the bus, in bed, etc. and make progress. If I had to spend the time and involvement of making the cards myself (which requires not only more time, but in practice a more dedicated setting with a PC and keyboard, etc., as I'm not going to make flashcards with my phone on the bus even if it's technically possible) I would probably have quit a long time ago.
So work on making the creation of flashcards easier seems like something that could be of interest at least to some people.
Creating a card is not just writing down some text (or, worse, copy-and-pasting it so that you remove all your agency entirely); rather, it's focusing a spotlight on a specific small aspect of your enormous mental model, determining how you can remember that particular aspect, and then writing down a card that will trigger you to illuminate that same spotlight in the future. Reviewing a card is not just dredging some detail from your memory; rather, it's remembering the reason you remember that card, and then re-activating the spotlight.
Writing and reviewing flashcards is a much more active process than many people think, and I am very wary of anything that purports to remove some of that activity, because the activity is what helps you remember!