I've always wondered why Vader didn't just blast the plenet to get a clear shot at the moon with the rebel base on it. 
If you paid attention in the dialogue, you would've heard the term "gas giant". Just like Jupiter. If they fired the big lazer, the gravity well of the gas giant would arc the trajectory and most likely miss the target. That's the scientific reason.
The real reason is bad guys always want to ramp up the drama allowing the good guys enough time to thwart the dastardly deed. Case in point. Each cliffhangar episode of Batman from the 60's.
The smart alec answer is this. The fire control guys were too busy playing sabbac and kept the "Broken Lazer" sign up on the outside face of the door. Plus the Twi'lek girls were dancing in the corner with the vertical pole attached from floor to ceiling. Who wants to leave that action to go out and push some little button? Let Vader do his own dirty work.
