Also if your application is shipped as orchestrated containers (like docker-compose), or as multiple containers in a 'pod' (e.g. sidecars), you may want the ability to run containers from containers as part of CI.
As of Molecule 3.0, at least -- they didn't in the past.
A while ago, I, unfortunately, decided to add "proper tests" to all of my Ansible roles and decided to use Molecule (2.22, at the time, IIRC). As I don't use Docker, I was using "lightweight VMs" I had created (w/ Packer, converted into Vagrant boxes) with VirtualBox for all of this testing.
I spent I don't know how many hours across several days learning the "toolchain", getting everything setup and working properly, adding full test coverage, and so on. Not long afterwards, they released Molecule 3.0 which required using Docker. :/
Also if your application is shipped as orchestrated containers (like docker-compose), or as multiple containers in a 'pod' (e.g. sidecars), you may want the ability to run containers from containers as part of CI.