Imagine I wrote a book. Four hundred pages, three months, my own life in it, every page mine. You are my senior editor. You read it before it can be published, and you come back and say: chapter fifteen does not make sense.
I fix it that afternoon. I know what is under chapter fifteen, because I put it there.
That is how software used to work. Every developer wrote every line by hand and carried the whole thing in their head, so a senior person could point at a page and get it fixed by the one human being who already knew that page.
Now run it again. The developer starts the agent and goes to sleep. The next morning there are eighty thousand books on the desk. The editor says something is wrong. What is wrong now. Nobody in that building has read eighty thousand books. You can read one book a day if you are superhuman. Assume you are the best editor alive and I hand you one of the eighty thousand, four hundred pages, and ask you to evaluate it in twenty minutes, because we are pushing it to production this afternoon. It does not matter how good you are.
And when I read my own book, I do not read the page. I read what I meant to put on the page, which is a different document, and it has no errors in it. So the editor cannot ask the writer, and the writer's memory, which used to do this job for nothing, left on the day the writing did.
That is where we are now. Agents write hundreds of thousands of lines a day. Reading them with human eyes, line by line, would take hundreds of years of human effort. The math does not close and it never will.
So the editor needs two things. A written statement of what the thing was supposed to do, made before anybody built it, by somebody who is not the thing that built it. A requirement is the sentence that says what correct means here. A test case is one checkable step that proves it.
Say you have that. Every chapter, written down in advance, by somebody independent. You still have eighty thousand books and one afternoon. So what has to change?
The reading, and this is the answer the whole book has been walking toward. Nobody reads eighty thousand books. So somebody turns those written statements into software that does the reading: a program that runs every check by itself, against all eighty thousand, overnight, and hands you a report in the morning saying which ones no longer match. That is test automation. The volume created the problem and automation is the only answer anybody has found to it, and the people who can build it are what the hiring is for.