The non-profit Cactus Lovers Association iN Galaxy (CLANG) considers its purpose the popularisation and cultivation of cactuses throughout the galaxy. (Heretics claim that this religious organisation is just a cover-up for the mighty, corrupt Galactic Cactus Corporation (GCC), but that will be none of our business for now.)
CLANG missionaries roam the galaxy, and provide everyone who desires with seedlings for Agave Tequilana, the, erm, cactus that provides us with the noble spirit that is Tequila. All they ask in return is a fair share of the Tequila produced. The canon says that the government who produces the most Tequila under their reign will be the first to rule the galaxy.
Now, all the people in the galaxy produce and collect Tequila. It turns out, however, that invading neighbours' planets and plundering their Tequila stockpile is a more efficient way to acquire juice. CLANG on the other hand looks away from the fights... Tequila doesn't smell, CLANG is happy when they receive their share, no matter from whom.
In more mundane terms: declare planets important by planting cactuses to produce score. Defend these planets because losing them gives you a penalty, conquering them gives a bonus.