should I use new keywords in laravel's controllers

I know DI, Solid Principles, factory patterns, adapter pattern and many more maybe. Now let's say I am creating a laravel application and it's gonna be huge. Let's say I have a postscontroller which is resource and has CRUD methods. Now Let's say in that controller's functions, I have a post Model and use it to retrieve data from database. I have a store function where I am creating new Post() and then put it into database.

1) Is it a good practice to have Post model directly in PostController's function and use new Post() also? What is bad in it? I know that this way I am not using dependency injection and patterns, but still why is it bad? As you know , I can still mock the object without dependency injection since laravel has so many amazing testing features. Then why is it that bad to write new keyword in controller's functions and also use Post model directly?