I'm guessing the location within the menu hierarchy is pretty well-distributed across the spectrum. And actually, I think it's pretty unusual for it to be used as a 404 page, usually it's for other purposes! Here are just a few examples:
1. Custom login page. (instead of using the Login control, you add the login module to a hidden page and then set it as the login page in Site Settings).
2. Custom Terms and Privacy Statement pages (create the pages and then hard-code the links to them in the skin, instead of using the Terms and Privacy controls).
3. Hidden return pages after shopping cart checkout in an ecommerce site (usually there are two, a return page after a successful checkout, and a failed order page).
4. The Search Results tab is probably by far the most common hidden tab in the DNN world, since it is included by default in most new installations and new portal templates.
As to your second question, how are you sending 404's to the hidden page? The answer would depend on that.