Why Testers Needed If Developers Them Self Can Test?

It takes a different mindset to create code than it does to test it. In short, a developer is trying to make it work; a tester is trying to break it.

Only an exceptional developer can do a good job of testing her own code. Often a developer isn't much good at testing others' code, either. A professional tester is familiar with test techniques, the usual (and unusual) errors in coding and/or specification, and with the tools that help set up, run, and document testing, assess its coverage, and estimate numbers of remaining bugs.