You are using a WP_Query object, you just don’t know it (yet).
WordPress generates a global called $wp_query which is WP_Query object. This object is what is responsible for what is called the “main Loop” colloquially. This $wp_query object is present on every page that I am aware of. Functions like the_post, have_posts, and some others, assume the $wp_query object. Take a look at the source for one of them:
775 /**
776 * Iterate the post index in the loop.
777 *
778 * @see WP_Query::the_post()
779 * @since 1.5.0
780 * @uses $wp_query
781 */
782 function the_post() {
783 global $wp_query;
784
785 $wp_query->the_post();
786 }
See how the function the_post simply grabs the global $wp_query object and then runs $wp_query->the_post exactly as happens when you created your own object.
It just so happens that there are functions matching the method names of some of the WP_Query object. Those are shortcuts to $wp_query and you can’t use them unless you intend to use $wp_query. That is why when you create a new WP_Query you have to use the long form– $my_query->the_post()