As shown we can use:

$friends = $this->follows()->pluck('id');

Or use:

$friends = $this->follows->pluck('id');

The final result in both cases will be the same. The difference will be the amount of data that will be fetched.

The difference is:

$this->follows() // returns the relationship object
$this->follows // returns the result of the relationship

When you execute:

$this->follows

You receive a Eloquent\Collection, of all User data present in the relationship. Depending on the number of followed users it can be a vast amount of data, including all attributes of users. Then you apply the pluck method to retrieve all of the values for the id key.

When you execute:

$this->follows()

You receive the relationship object, a Eloquent Relation, in other words you receive, as return, the object of the defined method:

public function follows()
{
    return $this->belongsToMany(
        User::class,
        'follows',
        'user_id', 'following_user_id'
    );
}

So at this point, as I understand, no data is fetched yet.

But why would someone do that? You usually would call the Eloquent Relationship or method object if you want to further specify the query before you run it. For example add a where statement, or like in our case the pluck method. So when the method is executed, it in the end will only fetch the id attributes of the follows relationship.

So as Jeffrey explains near 5:18, when we use:

$this->follows->pluck('id');

..."that's actually going to load everyone the current User follows, so you can imagine situations where a person is following five thousand people, this right here would create a collection of five thousand users".

And then when we chain the follows()->pluck('id') method we will execute a more specific query and return only the id´s, this will be more performant.

For reference in documentation you may see: Eloquent Relationships - Querying Relations https://laravel.com/docs/8.x/eloquent-relationships#querying-relations

It says: "Since all Eloquent relationships are defined via methods, you may call those methods to obtain an instance of the relationship without actually executing a query to load the related models. In addition, all types of Eloquent relationships also serve as query builders, allowing you to continue to chain constraints onto the relationship query before finally executing the SQL query against your database."

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