Leaving around plaintext username and passwords in a script on a box isn't the best from a security standpoint. This change ensures the scheduled task wrapper script for WinRM doesn't leave these around on the box, and instead passes them to the script as arguments.
Vagrant Core Plugins
These are plugins that ship with Vagrant. Vagrant core uses its own plugin system to power a lot of the core pieces that ship with Vagrant. Each plugin will have its own README which explains its specific role.