If you have a website, you need to have a 404 error page. That’s the page that loads up when the site can’t find whatever page you were just requesting.
However, there’s more to the page than that. You can customize your 404 page so that you can send your visitors to your main page or somewhere else, instead of just basically not giving them any other option except to click the ‘back’ button and go to someone else’s site.
If you provide your visitors with a website that’s user-friendly, even if they get an error page, they’ll be more likely to try something else in order to keep themselves on your site. You might gain a loyal customer that way, where you would not have done so if you would have used the standard 404 page that offers no real options for anyone who finds themselves on it.