Drupal Search 404 module is not a fundamental tool but an option to help guide users in your website.
The module replaces the normal 404 error page to a search engine. It performs a search for the keywords in the URL that the users tried to reach. For example, if they attempted to go to: www.example/drupal/is/cool and this page doesn’t exist, this module will search for the keywords “drupal is cool” and show them a list with the results related to these keywords, as oppose to displaying a normal 404 page.
This is a very efficient tool to keep users into your website and helps to retain the ones that are coming from old and expired links.