I plan to use this for testing changes to my boot units.
In theory, plain old generations allow you to safely test changes to boot units, by allowing you to jump to the previous generation. In practice, this involves remembering which generations have known-good boots.
Specialisations will allow me to run a stable and candy track, on per generation.
What other usecases do specialisations improve?
Another option is to use named profiles for this:
nixos-rebuild switch --profile-name whatever
The profile names show up in your boot menu.
In theory I could manage this with git rebasing and/or tagging, but in practice I lose confidence in whether I've accurately tracked.
With specialisations, I'd comfortably commit an experimental change to my canary track, even though it might break, safe in the knowledge that the stable track continues to boot.
Typos:
Specialisations will allow me to run a stable and canary track, one per generation.
It's candy for tinkerers.