Before there was vagrant, the days were dark and the nights were cold. Developers suffered from the "It works on my machine" virus. Weeks were wasted by setting up development environments, trying to reproduce the colleagues settings to finally start developing on that piece of software.
No more! Using a virtual machine to run the whole software stack and shared configuration for all developers, with vagrant it's possible to be as close to the production server as needed and get it up and running in under on hour - No matter how complex that stack is.