When the flaps are in other config than 0 is because the plane needs more lift, in order to help that or make it more efficient, the spoilers drop a couple of degrees to give the wing a more curved shape, therefore, the upper part of the wing can accelerate more air faster and create more lift out of it, that same ''aero technology'' is being applied on the 787, check it for your self at smartcockpit on 787 misc part, systems and performance page 22.
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