The $post global is set by $wp_query->the_post(). As $post is set by the main query and the fact that templates are set within the global variable scope, it is available throughout the template, so there is no need to explicitly calling the $post global when you are within the specified template as the WP_Post objects can be accessed any where. You can access the WP_Post objects by just simply calling them directly, for example $post->ID to retrieve the page ID on a page template
When you need to access post data outside of a template, for example, in a function or in a widget, you need to call the $post global to make the post data available for use.