It's not necessary, it's just an option...you could also pass a controller and method you want to handle the request. There are many many options when it comes to routing:
That is an anonymous function, and you can do whatever you'd like in there. The reason it's like that by default is because it may be a handy starting point to test stuff out.
Laravel also has a view() method on the Route facade, so you can clean it up by doing something like this:
// Register a GET endpoint at '/' that returns the 'welcome' view
Route::view('/', 'welcome');
It is also worth mentioning that most IDEs will not be able to index Laravel methods and classes, simply because the framework resorts to a lot of magic methods. I do however believe there are Laravel plugins that you can get for PHPStorm that will let you do stuff like this.
I think what you are referring to is a route closure that ships out-of-the-box. I think he's just illustrating a basic concept at that point within the series. Keep watching, he'll certainly follow up with best practices.