Getting stuff done
Even the best team runs out of the time or bodies necessary to get certain tests accomplished. If hiring a QA firm to perform certain tests takes care of some test goals, then outsourcing is a great idea.

Hiring expertise
Some tests are best performed by experienced professionals, either because the necessary knowledge requires specialize education or backgrounds, or because the necessary skill set can't be learned under a short deadline. Moreover, some tests require a specialized understanding in order to interpret and analyze the results, and it makes sense to outsource these tests.

Hiring authority
Some kinds of tests-and some kinds of test firm-have an authority that a homegrown QA team may not be able to compete with. Company executives often find it easier to accept critical results if they come from a third-party, rather than from their own QA team. Hiring a neutral point-of-view
Sometimes having an independent team perform testing tasks can provide an objective point-of-view about quality issues like usability or compatibility. Even if you are rigorously fair in your compatibility testing, you'll still have browser prejudices; you'll still have a favorite browser and platform, and so may tend to do the majority of your testing with them. Outsourcing some testing may provide you with more and unbiased data.

Hiring a fresh set of eyes
Involving a new QA team in testing exposes the code to new test tools and methods; a QA firm may find defects your tools were unable to find.