Why does Vagrant require dummy boxes with cloud providers?

Question

Apparently, you need a "dummy" box with Vagrant when you are going to configure cloud providers

• The provider for Azure v2 requires downloading a dummy box from GitHub
• The provider for GCE: config.vm.box = "google/gce"
• Ditto for the provider for AWS

There are some differences though, since the Azure box seems to actually be functional, while the GCE and AWS need to be superseded by issuing, within the Vagrantfile, the ID of an image to download.

The question is, why is this needed? Is it a hack to make cloud providers work with Vagrant? What is actually done with the dummy box in the last two cases? Is it actually uploaded and then ditched for the actual image that is provided?