Nobody’s perfect. And when it comes to web sites, no web site is perfect. Even with the best planning, it’s not unusual for a site redesign to result in there being a few broken links as the site structure is altered and old pages are removed.
In some cases, we’ll suggest a client keep old pages on the server to avoid a site visitor from being directed by a search engine to a dead page. But, that’s not always possible, in which case, it’s important to have the proper redirects set up.
Inevitably with a larger site that’s been online for some time and gone through several iterations, dead links will occur. In which case, it’s important to have an optimized Error 404 page.
The value of an optimized Error 404 page is that the site visitor has an opportunity to click on a link from a navigation bar to get to the main site and also to learn a little about the site, even though it’s technically a dead page. Also, the search engine spiders will be able to continue spidering your site via the links in the navigation bar.
Here’s a good article on how an optimized Error 404 page is set up.