Allow per-minion configuration of the image that will be used to set up ...

...the minion.
It is particularly useful to boot a Kubernetes cluster that is heterogeneous because then you can test the initialization process and the integration points in parallel for multiple distributions and without paying for two full clusters.  I use it to test CentOS SaltStack automation simultaneous to testing Debian SaltStack automation, and I expect others to be able to do the same for their favorite distribution.