The WP_Post object's main function is to contain post objects that are stored by the database. It's returned by functions like get_post().
The WP_Post object contains information about the post. This includes post ID, the author, post date, comments, content of the post, and quite a bit more.
WP_Post's role in page load is to gather the information about the post(s) and have it ready for retrieval.
The WP_Post object is quite straight-forward. There's not a lot to it, it's role is to contain the post data and have it readily available for debugging purposes.