What are differences between SystemJS and Webpack?

I'm creating my first Angular application and I would figure out what is the role of the module loaders. Why we need them? I tried to search and search on Google and I can't understand why we need to install one of them to run our application?

Couldn't it be enough to just use import to load stuff from node modules?

I have followed this tutorial (that uses SystemJS) and it makes me to use systemjs.config.js file:

/**
 * System configuration for Angular samples
 * Adjust as necessary for your application needs.
 */
(function(global) {
  // map tells the System loader where to look for things
  var map = {
    'app':                        'transpiled', // 'dist',
    '@angular':                   'node_modules/@angular',
    'angular2-in-memory-web-api': 'node_modules/angular2-in-memory-web-api',
    'rxjs':                       'node_modules/rxjs'
  };
  // packages tells the System loader how to load when no filename and/or no extension
  var packages = {
    'app':                        { main: 'main.js',  defaultExtension: 'js' },
    'rxjs':                       { defaultExtension: 'js' },
    'angular2-in-memory-web-api': { main: 'index.js', defaultExtension: 'js' },
  };
  var ngPackageNames = [
    'common',
    'compiler',
    'core',
    'forms',
    'http',
    'platform-browser',
    'platform-browser-dynamic',
    'router',
    'router-deprecated',
    'upgrade',
  ];
  // Individual files (~300 requests):
  function packIndex(pkgName) {
    packages['@angular/'+pkgName] = { main: 'index.js', defaultExtension: 'js' };
  }
  // Bundled (~40 requests):
  function packUmd(pkgName) {
    packages['@angular/'+pkgName] = { main: '/bundles/' + pkgName + '.umd.js', defaultExtension: 'js' };
  }
  // Most environments should use UMD; some (Karma) need the individual index files
  var setPackageConfig = System.packageWithIndex ? packIndex : packUmd;
  // Add package entries for angular packages
  ngPackageNames.forEach(setPackageConfig);
  var config = {
    map: map,
    packages: packages
  };
  System.config(config);
})(this);

Why we need this configuration file?
Why we need SystemJS (or WebPack or others)?
Finally, in your opinion what is the better?

If you go to the SystemJS Github page, you will see the description of the tool:

Universal dynamic module loader - loads ES6 modules, AMD, CommonJS and global scripts in the browser and NodeJS.

Because you use modules in TypeScript or ES6, you need a module loader. In the case of SystemJS, the systemjs.config.js allows us to configure the way in which module names are matched with their corresponding files.

This configuration file (and SystemJS) is necessary if you explicitly use it to import the main module of your application:

<script>
  System.import('app').catch(function(err){ console.error(err); });
</script>

When using TypeScript, and configuring the compiler to the commonjs module, the compiler creates code that is no longer based on SystemJS. In this example, the typescript compiler config file would appear like this:

{
  "compilerOptions": {
    "target": "es5",
    "module": "commonjs", // <------
    "moduleResolution": "node",
    "sourceMap": true,
    "emitDecoratorMetadata": true,
    "experimentalDecorators": true,
    "removeComments": false,
    "noImplicitAny": false
  }
}

Webpack is a flexible module bundler. This means that it goes further and doesn't only handle modules but also provides a way to package your application (concat files, uglify files, ...). It also provides a dev server with load reload for development.

SystemJS and Webpack are different but with SystemJS, you still have work to do (with Gulp or SystemJS builder for example) to package your Angular2 application for production.