Having hard time understanding Forms from laravel collectives

Question

I'm new to Laravel. I've created a form and trying to delete one of the post. I just want to know how opening a form from Laravel collectives works.

Currently, this is how my form looks like in show.blade.php

   {!! Form::open(['action' => ['PostsController@destroy', $post->id], 'method' => 'POST', 'class' => 'pull-right']) !!}
    {{ Form::hidden('_method', 'DELETE') }}
    {{ Form::submit('Delete', ['class' => 'btn btn-danger']) }}
    {!! Form::close() !!}

The above form gives me the error Action PostsController@destroy not defined.

But when I add 'url' => 'posts/' in Form i.e.

    {!! Form::open(['url' => 'posts/', 'action' => ['PostsController@destroy', $post->id], 'method' => 'POST', 'class' => 'pull-right']) !!}

The above error disappears.

Below is the destroy() function in PostsController

class PostsController extends Controller
{
    public function destroy($id)
    {
        $post = Post::find($id);
        $post->delete();
        return redirect('/posts')->with('success','Post removed');
    }
}

web.php

// --- Posts Routing
Route::get('/posts', [App\Http\Controllers\PostsController::class, 'index'])->name('posts');

MY QUESTIONS

• I'm redirecting in the destroy() function in PostsController, why url is necessary with action in Form from Laravel collectives to avoid the above error?
• When to use 'url' and when to use action?

I've laravel 4.2.10.