The subfolders could be used to categorize notes. That would be useful for project management notes for a project that is no longer active.
This request pops up from time to time, but we're postponing to discuss this until version 2 of the app. The original intention was to nudge users to focus on 1 flat folder and not categorize at all, for the best possible Zettelkasten experience, so to say.
(Edited the answer when Sascha pointed out I left out the most important information)
Author at Zettelkasten.de • https://christiantietze.de/
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