WP_Post objects are the objects that are stored in database by WP_Post class and are returned by functions like get_post.
WP_Post data contained in the object: ID, post_author, post_type, post_title, post_date, post_date_gmt, post_content, post_excerpt, post_status, comment_status, ping_status, post_password, post_parent, Post_modified, post_modified_gmt, comment_count and menu_order.
The role of WP_Post object is to store post objects to be accessed by methods such as get_post()
In summary, WP_Post is used to store the data about a post in order to be echoed anywhere on the website.