A 404 error page, No one likes it. A 404 error appears when a user clicks on a link or enters a URL which is no more exist. It may be because the page is deleted by the webmaster or it may be because the user have entered a wrong URL.
When a someone gets a 404 page, they don't have any option but to close the tab and which means you have lost that visitor. By creating a custom 404 page you can give your reader a better option.
Having a customized 404 page provides reader a better user experience.
A good 404 page can help in reducing bounce rate.