Let me show you the job from the inside, on my own software, because I do this work and not only talk about it.
A few weeks back I was adding the library to our learning system. New pages, new buttons, a new thing students can open. It sat on its own copy of the software, off to the side, where it could not touch anybody. We call that a branch, and at that point I had seventeen of them.
The full set of checks on that system is four or five hundred test cases and takes about four hours to run. I was not going to wait four hours on a branch. So I ran the smoke tests first, which is a small set that only asks whether the thing is standing up at all, and within about half an hour I knew the library worked and nothing obvious around it had fallen over.
Then I moved it to the QA environment, which is a full copy of everything, where the checks can run for real. And there I ran all four or five hundred.
They passed. Once all pass now you can deploy, and that is the sentence the whole job turns on. It went to production, the live one, the copy with real people in it, and the next morning a few thousand students logged in and the library was simply there, the way a door is there.
Somebody complained that day. Not about a broken page. He did not like the book I had put on the shelf.
Four hours, on a feature I had already watched working with my own eyes. So what was I actually buying?
That complaint. A man telling me my taste in books is bad is the most expensive thing that happened that day, and four hours is what it cost to make that the worst outcome available. Nobody thanks you for it, because nothing happened, and nothing happening is the entire product. That is why this work is invisible from the outside, and why it is the first thing an impatient company cuts. The four hours never show up anywhere. The morning they get skipped does.