We need to speak about page 404 from two perspectives: from the point of view of users and from the point of view of search engines. Page 404 is called so because when you access it, the server returns an error of 404, i.e. message that the user tried to go to a non-existent page. If the server's response is correct, then from the search engines' point of view there are no problems: an appeal has occurred - the answer is received. But at the same time, the user can see a white screen with an error number, an empty page of the site or a site map, and if the site is compiled on a template, even forgotten pieces of test content can be seen.