Specifying Middleware Within Controller's Constructor - laravel

Question

Here's a code snippet from laravel in-built RegisterController.
public function __construct(){
$this->middleware('guest'); //What does it actually do?
}
I know that it's a good practice to define middleware in the
controller constructor but I need to know what
$this->middleware('guest') actually does and what parameter (where
?) it sets.
Second question: within the same controller (RegisterController), we use RegisterUser (defined in namespace Illuminate\Foundation\Auth) but it seems we never use it throughout the controller (overriding methods or attributes). I'm a little bit confused. Thnaks in advance!
class RegisterController extends Controller
{
use RegistersUsers; //?????
/**
* Where to redirect users after registration.
*
* #var string
*/
protected $redirectTo = '/home';
/**
* Create a new controller instance.
*
* #return void
*/
public function __construct()
{
$this->middleware('guest');
}
/**
* Get a validator for an incoming registration request.
*
* #param array $data
* #return \Illuminate\Contracts\Validation\Validator
*/
protected function validator(array $data)
{
return Validator::make($data, [
'name' => 'required|string|max:255',
'email' => 'required|string|email|max:255|unique:users',
'password' => 'required|string|min:6|confirmed',
]);
}
/**
* Create a new user instance after a valid registration.
*
* #param array $data
* #return \App\User
*/
protected function create(array $data)
{
return User::create([
'name' => $data['name'],
'email' => $data['email'],
'password' => Hash::make($data['password']),
]);
}
}

Answer 1

1) $this->middleware(...) called in a constructor of a controller is only adding what ever value is in ... to an array named middleware on the controller. That is it at that point. Nothing is "ran" at that point. You can see this functionality in the Controller your Controllers extend from, Illuminate\Routing\Controller.
Later when the Router needs to dispatch the Request it will build a middleware stack to pass the request through by gathering the middleware which includes asking the instance of the controller for its defined middleware via getMiddleware.
public function getMiddleware()
{
return $this->middleware;
}
2) The trait is including functionality and variables into your controller. There are routes pointing to some of these methods on the controller.
You could take all that code and paste it into your controller and it would be the same thing basically.
If you need to change any functionality of that controller or customize it you will end up redefining some of those methods or adjusting the variables.