Checked, not assumed
A change isn't done because it looks finished. It's checked before it goes live, so nothing goes out on a guess.
Actually tried out
Looking something over and assuming it works isn't enough. We also try it out, using it the way you really will, so something that looks right but doesn't actually work doesn't slip through.
If it can't be shown to work the way you asked, it doesn't go live.
Why it matters to you
The result is simple: what you get back is meant to be used, not fixed up by you first. That's the whole difference between a rough draft and software that works.
