Waste made visible
Bad campaigns are easy to find once they have spent enough. The expensive ones are the campaigns that look fine — spending steadily, converting rarely — and the ones that stopped working without stopping costing.
Each campaign against its own target
Cost per acquisition is not one number for a whole account. A campaign carries its own target CAC, or inherits its market's, and it is judged against that rather than against a portfolio average.
Not enough data is not the same as bad
A campaign with two days of history and a campaign with two weeks of failure look identical in a CAC column. They are told apart here, and only one of them gets acted on.
It notices a campaign that broke
A campaign that was never shown, that stopped being shown, that is shown and never clicked, or that stopped converting while still spending — none of those show up as bad CAC, because a campaign that does not spend has no CAC to be bad. They are found and named.