ng add
ng add <package> uses your package manager and installs the dependency. That dependency can have an installation script which can be used to do more job except the dependency installation. It can update your configurations, download another dependencies based on that one or create scaffold templates (with initial markup and logic).
To use ng add for a third party dependency, that team must provide schematics which describes the installation script. This can include some .scss or .css or related .js files to be included in the angular.json file.
In your provided link, you can install material package and also create some components with components
npm install
npm install <package> just installs the dependency.
For more Version 6 of Angular Now Available.