Let me ask you this question. How many times have you made changes to the page structure of your site? Ten times? 100 times? 1000 times? If you have a large site, the number of changes can grow rapidly. This is especially true if you deal with dynamic sites that are driven by a complex scripting language and a database.
Now let me ask you one more question. How many times have you “fat fingered” the keyboard when typing code? I cannot speak for anyone else, but I do it all the time. Now, what happens when you are coding hundreds or thousands of lines of code? Are you going to catch that one little mistake, or are your clients going to catch it?
So, what happens when a link is no longer active on your site, or when you “fat finger” while typing your code and a link cannot be found? Error pages are generated. The most common of these is the 404 error. This error is generated by your web server software to let the end user know that it cannot find the file specified. The big issue here is that this page does not include a link back to your site. And unless you are the server administrator, it may not include your email address, in which case...