In Star Wars: The Last Jedi the following happens at the start of the film: The First Order decide to aim the Dreadnought cannons at the rebel base. As they point out, the rebels are in the middle of an evacuation, and are heading from the surface up to the cruiser.
Why make this decision? Would it not have been more tactical to destroy the cruiser first? Surely this would have foiled any attempted evacuation, barring some much smaller frigates and fighters which would probably be much more vulnerable without the protection of the cruiser.
Why did they make this decision?