I am not sure if this is what you want to know, but:
- Debian.org or Ubuntu.com officially does not support any of those boards/boxes,
- Armbian userspace has many small but vital performance or security adjustments,
- Armbian fancy some kernel development and a lot of its maintaining. Debian relies on upstream sources for ARM hardware which can be years behind and/or lack of many functions,
- Armbian userspace is lean, clean but 100% Debian (or Ubuntu) compatible
- many stock Debian bugs are fixed on the way, "better than original :)"
- a build system is a central part of this whole ecosystem. You can DIY. Debian much harder.
- dedicated support forums per boards/boxes
- plug and play vs. complicated install scenarios on Stock Debian
- unified development scenarios and user experience vs. mess of different setup instructions scattered all around