Power
If you're on a costuming team but are mostly working to bring the head designer's vision to fruition, then you don't have any power—you're basically a lackey with needle and thread. If you're the head designer, you have only slightly more power—the power to envelop audiences in the world of the story for a couple of hours with your beautiful and 100% accurate and realistic costumes. But it's not like you have the authority to declare war or anything.