There’s some more explanations of how disk serial number support is working from Matthew Dillon, plus a warning that a full kernel/world rebuild is needed because of these changes.

If I’m reading it right, serial number support, combined with a dynamic /dev, makes it possible to identify a disk by serial number, assign a name to it, and then refer to that disk directly by name in places like /etc/fstab.  Much, much easier than remembering /dev/ad0c or /dev/ad1a, and so on.