Think of both as ways to get a consistent baseline system which your configuration will be applied to. It's faster to spin up a VM or container than it is to do a fresh install on bare metal so you can realistically test that you really could do a clean install from the documented starting point.

Containers add even faster startup time and the option of isolation so you can have multiple containers which think they're writing to the same location but actually aren't. That's not terribly important in a one-app-per-system model but it's awesome for testing or running multiple instances of an insufficiently-configurable app on the same system.