Thats true. But what if your APACHE doesn't allow you to change error pages? For example I have a free PHP host that allows .htacces and mod_rewrite but adding 404 pages just dissapear because they have they're own. Using mod_rewrite you can still achieve the same result.
Also if you don't have APACHE at all, or don't have access to it you can still use it if you have a dynamic include functions. If the file doesn't exist, you simply include the proper error instead.
There are plenty of ways to use this without APACHE as even some scripts have they're own defined 404 pages which can be altered to this script as well.
The only problem with this is, you don't really need the manager considering if you are doing things for the site it would usually only be a 404 error, all other errors will still be handled by the server if you don't have APACHE and define the new pages.