How can you be certain that your disc content is going to be perfect?
It's simple, you have to see the replicated disc. (there's a pattern here)
Doing audio / music cds? Musicians like to know that no one has tampered with their master and bumped up the volume. Wouldn't it be a good idea to listen to it and approve it before thousands show up?
Doesn't it sound a little foolhardy to ship them without listening to one?
Doing software / cd-roms? How about giving the final disc a test drive on a number of different computers?
Doing video / dvds? Whoo boy. Do you know that recordable dvdrs can play differently than pressed dvds? Yep. That means that the master you send in may not play the same as your pressed dvd. Think a test disc would be a really good idea? The ISO standard for dvds and dvd players/drives/computers is still changing to keep up with new authoring techniques and technology demand. If you're old enough (sshhh) then you remember this happening with recordable CD-Rs when they first came out. Recordables wouldn't always play in a home stereo or a computer or (gulp) at the radio station.
Machine testing.
Don't we have machines that compare the client master to the replicated disc? Yep.
Don't we have machines that make sure the masters are ISO compliant? Yep.
Don't we have machines to test for manufacturing defects to make sure the replicas are in good order? Yep.
But does all that machine testing make it perfect? No...because perfect is defined as how you want it to be, and the machines can't know what's in your head (well not yet, and maybe not ever). Anyway, that's why only you can say if it's perfect. We can check for technically correct, but "perfect" comes from you.