Spoilers Star Wars: Andor season 2

[Mustrum_Ridcully]

This is very true, but @Retros_x has a solid point. The Empire may have a great number of supporters on a planet, and it would be illogical to destroy them along with Rebel elements (though totally on-brand). Additionally, planets have raw materials and resources which can be useful. Blowing up the planet destroys those resources. The Empire can commit genocide in many other ways.

But there are countless of planets with resources, and blowing up a planet also gives you access to plenty of resources, stuff hidden in the mantle or core that you now expose to asteroid mining. And there aren't that many planets with uppity rebellions, but plenty with resources.

But the primary purpose of the Death Star would presumably be to be a standing threat to ensure that the great number of supporters will always keep the Rebel elements in check because they don't want to be blown up. No ground troops that you could try to defeat, no enemy fleet you could try to battle with your own ships. Just a machine so powerful that only an Empire can build it, no planet can hope to rebel. "They can't kill us all" might be true, but technically, they definitely could - and do you want to be the one that's killed?