When they don’t have an automated suite of tests acting as a safety net, the programmers may start viewing the testers themselves as a safety net. It’s easy to imagine that Joe Programmers’s thought process goes like this: “I ought to go back and add some automated unit tests for formatEmployeeInfo, but I know Susie Tester is going to check every page where it’s used manually. She’ll see if anything is off, so I’d just be duplicating her effort.”
It’s nice that a programmer would think so highly of the tester’s talents, but Joe is headed down a slippery slope. If he doesn’t automate these unit tests, which other tests might he skip? Susie is going to be awfully busy eyeballing all those pages.
Teams that have good coverage from automated regression tests can make changes to the code fearlessly. They don’t have to wonder, “If I change this formatEmployeeInfo module, will I break something in the user interface?” The tests will tell them right away whether or not they broke anything. They can go lots faster than teams relying exclusively on manual testing.
Source: “Agile Testing”, Lisa Crispin and Janet Gregory, Addison Wesley