is necessary to use guard in laravel code example

Example 1: laravel guard

They arere the definition of how the system should store and retrieve 
]information about your users.

You can find the configuration in your config/auth.php

https://stackoverflow.com/questions/34896130/laravel-what-is-a-guard
https://mattstauffer.com/blog/multiple-authentication-guard-drivers-including-api-in-laravel-5-2/

Example 2: where is the guard setting in laravel

You have to also register the guard in the config\auth.php

'guards' => [
    'web' => [
        'driver' => 'session',
        'provider' => 'users',
    ],

    'api' => [
        'driver' => 'token',
        'provider' => 'users',
    ],

     'admin' => [
        'driver' => 'session',
        'provider' => 'admins',
    ],
],