Because of what you can capture on them. For instance, there are special structures of WAR that you can seize that can give you bonuses in later missions. Seizing a communications array on a planet gives you access to the Signum, which lets you call in a powerful artillery strike in a mission. It's so powerful that you can only use it once per mission, but if you control multiple communications arrays on a planet, you can call in multiple strikes.The problem is that a map might feature a foundry and a communications array, and you can only seize one of them.  But then there are also other structures like automated foundries that heal and deploy units faster. You must make a choice. Not only that, but if the enemy recaptures the special structure later in the campaign, you may make a choice to revisit the map to take it back.

The rest of the campaign consisted of traveling between two planets to battle Ork armies, defeating their commanders (they have colorful names like Skykilla and Gutrencha), and discovering that all hell is breaking loose in your corner of the galaxy.It certainly helps that your squad leaders and your commander get more powerful, as you can improve their firepower, boost their health, equip them with exotic weapons and equipment. It almost feels a bit like X-Com, the classic turn-based strategy game that started you with weak characters but by the end of the game you commanded elite troops.  It makes for an addictive experience because you get caught up in the action.