Admiral Ackbar: Although the weapon systems on this Death Star are not yet operational, the Death Star does have a strong defense mechanism. It is protected by an energy shield which is generated from the nearby forest moon of Endor. The shield must be deactivated if any attack is to be attempted. Once the shield is down, our cruisers will create a perimeter while the fighters fly into the superstructure and attempt to knock out the main reactor.
What is the use of creating perimeter after shield is down?