为什么保护用于模型Laravel

I am new to Laravel and as I was watching beginner's course of Laravel an author used protected in a model that he created. Here is the code:

class PostsAdmin extends Model
{
    protected $table='posts';  
    protected $primaryKey='id'; 
}

So, it is unclear for me why to use protected in above cases. Guys I understand the meaning of protected but I cannot understand why protected is used in model in Laravel .