What is the site error 404 page's purpose?

The File Not Found Error page (or Site Error 404 page) appears whenever a visitor enters an invalid URL for your site. Invalid URLs can occur if the user types in the URL incorrectly or if you have broken links on your web pages, resulting from a page being renamed or deleted.

If your account is one that has a page limit, the siteerror404 does not count towards your pages.

Deleting this page can actually confuse Google bots and could cause them to fail indexing your site correctly, so we highly recommend not deleting this page.

In fact, Google also suggests customizing this page with links to other popular pages on the site, or some humor. You can do this from within Sitebuilder.