The WP_Post object contains post objects stored by the database. Functions such as 'get_post' are returns for WP_Post.
Data stored in WP_Post object includes: author, post type, post title, date, content, excerpt, comment count, and more.
WP_Post's role in the page load is to store post objects so they are ready for access by methods such as get_post(); echo $example->post_title;.
In summary, WP_Post is used to store all information you need about a post so that you can choose what to echo onto your site for visitors to see.