The 404 page should be as light as possible, or ideally nonexistent. When the browser requests a resource which does not exist (image, CSS style sheet, JavaScript file, etc.), the server replies with the 404 page, which is sometimes larger than the resource requested.
Moreover, some CMS run their content research process (in the database) in an attempt to retrieve the requested page. This results in the server code being run, the server’s database accessed and the HTML page being dynamically generated. All these processes waste CPU power, RAM and bandwidth.

Browsers allow users to quickly navigate between pages in their history (Back and Forward buttons). This features avoids the need to request and download the page from the server for a second time. This prevents consuming unnecessary bandwidth and generating additional HTTP requests.
It is therefore recommended to not implement elements that render the page unoperational after leaving it.