Nothing at all really. The cloud is just a convenient way to transfer documents and notes (but you can still do so over USB).

The only thing that really needs the cloud service is transfer from and to mobile devices, which is an understandable niche. The Remarkable does not act like a regular USB drive. Instead, when plugged in, it acts as a virtual network device, and you browse to it on a browser, uploading and downloading documents via a browser interface. This behaviour doesn't seem to work properly on Android and Apple sure as hell don't allow it on iOS.

If you really must have direct access to the files and OS, it allows for SSH access as root, and provides a surprisingly full featured Linux environment. If you're the experimenting type, you can even put homebrew applications on the device, and it has a modest homebrew app community. Just...be really fucking careful not to bork the OS to the point SSH doesn't work, else you're fucked unless you wanna tinker at the hardware level. Also, direct access to the document files isn't as useful as you'd think because their internal filesystem is confusing as shit. You're always better off using the device or cloud web interfaces.