It's a numbering consistent across boot, and consistent with u-boot's device numbering, which means the boot script can first load a kernel from mmc $n partition $p and then pass root=/dev/mmcblk${n}p${p} to the kernel.

 

I don't see how that is helpful at all. If I just wanted some DT-specified label I can slap some property onto the DT nodes and access it via the of_node symlink. This is e.g. what I do for uio devices on beaglebones:

SUBSYSTEM=="uio", SYMLINK+="uio/%s{device/of_node/uio-alias}"