Whether 404 custom error page necessary for a website?
My answer to Whether 404 custom error page necessary for a website? on Quora
They aren’t required, but if you don’t have a custom 404 page you’re missing out on a very easy way of improving the user experience of your site, and protecting against expired or incorrect links from elsewhere on the web.
Even just a search box and a link to your homepage is enough to ensure visitors who arrive on a 404 can still visit the rest of your site, and hopefully find what they were looking for when they clicked on the link.