If you mount docker.sock you're building on the runner's daemon so you have to worry about builds interfering with each other. If you use docker-in-docker you get a clean environment every time.

If you don't manage your own CI server then that is not an option is it?

I also never said that it requires two levels of Docker or that it requires DinD. I was responding to the general question of the parent who was asking a question of someone who was running DinD. I responded to the GP below that Kaniko also solves this problem so clearly I'm not advocating for running DinD or that this is even needed.