Vagrant works by describing a machine or group of machines. It hands off to Chef or Puppet, which then do the job of bringing the naked machine up to something nearly identical to your production environment.

You can snapshot to binary blob too, but having code that rebuilds from scratch is a great way to smoke out certain classes of dependency conflicts. It also provides assurance that if a hurricane turns some data centre into lego bricks, you can rebuild an identical setup somewhere else with a single command.