Confused about iCloud and iCloud Drive

System Preferences→AppleID, 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 sync to other devices like iPad or iPhone, but what data does Email have to sync with? Cause Email can retrieve emails from email servers directly, it does not need to be sync, 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 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 too(build-in App), which is similar to Dropbox, Apps like Calendars, Contacts, Reminders, Notes and so on are all sync data respectively, 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

Ticking the box for Mail means that you are using and accessing the iCloud email servers for Apple accounts.

By default, the iCloud email servers use IMAP, which does sync its contents with one or more email clients, allowing you to have the same emails on multiple devices.

The second option is, as the dialog suggests, to allow Mail to store documents within iCloud file storage.