Because in it, you'll be tasked with blowing up the Death Star. Called Battle Station, this new mode unfolds over three phases with Imperials and Rebels fighting it out in space, aboard the Death Star, and finally back out in trenches as they look to destroy the Empire's ultimate weapon. Spoiler alert, there's a weakness with one of the thermal exhaust ports.
Phase 1 of Battle Station sees Rebels in X-Wings and A-Wings fighting against Imperials TIE Fighters and TIE Interceptors around a giant Star Destroyer. The goal for the Rebels here is to destroy enough enemy fighters for Y-Wings to come in and do their thing.
Phase 2 takes place aboard the Death Star, with Rebels tasked with extracting an important droid of sorts. This Phase will also include the new heroes, Chewbacca and Bossk.
Phase 3, the last one, sees the Rebels tasked with taking out the Death Star. And in order to do that they'll need Luke Skywalker in his Red-5 X-wing. Of course on the Imperial side, Darth Vader will be in his special TIE Advanced. Saying stuff like "The Force is strong with this one" and "I have you now!".
The Death Star expansion will become fully operational in September.