Their primary duty is to try to break into customer networks to test the security. A problem arises though, when these big companies have a professional image to maintain. They can't be seen as hiring 'hackers' sometimes, or if they can let that term slip, they must qualify it with 'ethical' or 'white hat' hackers. In the past year or two, these claims have gotten completely absurd and backfired on some companies. In the struggle to keep the corporate image, some of the press about these teams has lead to outright lying about who they hire.
Lets look at two examples of hackers applying for a job with one company and the potential results.
Joe White Hat applies to Large Security Firm. He is applying for a position as a penetration tester. On his resume and in interviews he only talks about his experience as a network admin, doing security for his friends, and other white hat type stuff. He has no criminal record. John Black Hat applies to Large Security Firmm.