System Preferences > Apple ID, there's an App list that using iCloud on this Mac
Note that Mail and Reminders app are on the list, I can understand the data of Reminders needs to be synced to other devices like the iPad or iPhone, but what data does Email have to sync with? Because email can retrieve emails from email servers directly, it does not need to be synced, right?
Let's continue, click the Options... button of iCloud Drive, it shows up an App list too, Apps that store documents and data in iCloud, again, we can see Mail and Reminder in the list, but what's the difference of Mail and Reminder between this list and the list mentioned above? What's more, I can only check the Mail above but uncheck the Mail on this list, but it does not make any difference, I can't see any of Mail's data on my iCloud Drive(neither on my Mac nor the web page side)
In my opinion, I think iCloud Drive is an app (build-in App), that is similar to Dropbox, Apps like Calendars, Contacts, Reminders, Notes, and so on all sync data, not through iCloud Drive, if I need to sync files through iCloud Drive, I just need to save files to iCloud Drive folder, that's all.
My System Version: macOS Big Sur 11.5.1