This is usefull when we are testing. If we have a relationship defined, and we don't want a DB query to be runned we can just call the setRelation() method.
/** @test */
public function some_test_function()
{
$user = factory(User::class)->create();
$article = factory(Article::class)->create(['user_id' => $user->id]);
// will run DB query
$user->article;
// will NOT run DB query
$user->setRelation('article', $article);
}