There's two things here.

The first is layering. A goal is smarter layering without a Dockerfile. A dependency-management tool gives fairly good insight into how to arrange layers to improve cacheability and reduce layer churn.

The second is security. Going without a daemon socket is more than "pretty nice", it is a massive security win. Docker daemons have a lot of privileged access and a wide API. Having that entire massive attack surface lying around to write some tarballs and a JSON file on your behalf doesn't make a great deal of sense.

These both become more valuable at large scales, which is why it makes sense for Google folks to have developed it.