I think your argument is logical, as if we want to make cards! Why not make them in anki itself as it also has sync features. But it comes down to preference, You see if you want to make a centralized workflow for your knowledge, you can do just that with anki and obsidian plugin, some implications might be that
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You are reviewing your subject notes or any notes, and you come across valuable information and want to jot it down quickly into a flashcard without opening anki, as it decreases the time to make a flashcard and reduces friction.
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You want to have a seperate knowledge base for your Questions so you can use them else where, as it is markdown so you can open it besides the anki software, which makes it convinient to work with different softwares!
Personally I also had the same question myself when I was starting out with the Obsidian and anki plugin, then I realized that I could use the toggle feature in the obsidian software so that when I see the question, or flashcard in obsidian, I actively think about it and try to retrieve the information! so I basically donot use the plugin unless it is something I want to store in Obsidian as well as Anki. After that I toggle down the question and see the answer!!
Hope it helps you understand the Usage!