Besides the obvious reason 'Quality', I feel there are several other vital tasks performed by the quality department that no other team can perform quite as efficiently.

 - The quality team functions as the 'internal customer' for the organization. The issues identified and resolved by this group saves the company's image and reputation which otherwise would have ugly consequences had the real customers discovered the issues. Having an internal requirements to confirm to really helps.

 - Reporting! This is something that the upper management cant do without. Converting all that data into information is on of the other important responsibilities of the QA team. Preparing reports with emphasis on the  relevant data for the right people is an art in itself. These reports are so powerful that they even determine the future funding for the development teams.