laravel - why function call with no parentheses?
Laravel uses the magic function __get to handle arbitrary attributes.
This calls Illuminate\Database\Eloquent\Model's getAttribute function, which checks the model's relations and returns the related item(s) if a relationship is present with that name.
The parentheses are not needed because getAttribute automatically executes the function items() when the attribute items is requested. You can, by the way, request Auth::user()->item(); which will return a query builder you can work with.
The method item() is setting up a relationship for the Eloquent ORM on how to prepare a query. calling ->item is telling Eloquent through its Dynamic Properties that you want Item and then Eloquent will use the method. You can only call the method directly if it is compatible with Query Builder. The example you give should work either way but there may be something I am missing.