Debra is an advisor to the president. before a weekly staff meeting, the president asks her to play ‘devil's advocate', and argue against the majority opinion. the president is most likely trying to prevent from occurring

The President is trying to alleviate groupthink. Having Debra take an opinion against the majority allows the rest of the staff to re-think their own positions and see the merits (and demerits) of both sides of the argument. This stops everyone from blindly falling into a mode whereby they will agree with anything posited, simply because someone else put the idea forward.