I think docker is about ease of deployment. You still use the same server you just run a different command. As opposed to creating a new image and spinning up said image.
You can imagine roll back / migration is pretty easy in this case.
You don't get that with physical hardware unless you want to overpay. We could overpay for depreciating plant assets or we could overpay for variable costs that we can more directly control. The latter makes sense to us.
That said, I use Docker at home too[1] because it does make thinking about things easier. I dump my blog in a thin container not because I urgently desire security (though with Wordpress I kind of do worry...), but because it lets me develop and deploy using the same tools.
[1] - http://edcanhack.com/2014/07/docker-web-proxy-with-ssl-suppo...