Why is a 404 not found template important?
Adequate maintenance of 404 errors ensures a smooth user experience. It's quite inconvenient and somewhat annoying for a visitor to land on a dead page, after all, they are looking for particular content on that page and the page cannot be loaded. There’s a good chance they'll leave the website and search for a competitor's website that can provide them with the information they're looking for.
In order to maintain not found pages, the 404 errors are generally logged on the webserver. This log not only logs the not found page, but usually also the referring website and therefore the page where the dead link is placed. It's the webmaster's responsibility to monitor the 404 log regularly, to check if the internal links (within the website) and external links (from other websites to the own website) are all working correctly.