Not a sphere, a circle. It’s the shield they put between the stages so that they can do hot staging, where they light the second stage engines before the stages have separated. The exhaust from the second stage engines needs to deflected away from the liquid oxygen tank of the first stage. They ditch it after staging because it's heavy and the first stage has very little fuel left for the landing. Also probably for stability during the descent, which requires that the center of mass be as low as possible.
