Well, was looking for the same answer but couldn't find any that is satisfying so , if your also unsatisfied just like me try running the link returned when you run
php artisan serve
it returns
Laravel development server started: <http://127.0.0.1:8000>
copy that /http://127.0.0.1:8000 and run it into the browser , guess what it returns );the page that u first got when you installed laravel for the first time or i guess it will return the page in the routes folder which was set as /home directory or file(default home page).
In brief:
php artisan serve
starts the serve,forexample its like when your going to drive a car and you start the engine before driving whereby you can start the engine and drive at the same time ,its not neccessary to do so but depends.So to me that's php artisan serve CLI.