Designations are only part of the story.Because of designations.
Buildings that buff Job X are more cost-effective on a planet with 40 of Job X than a planet with 20 of Job X.
So, it makes more sense to build one planet that makes Alloys and is stacked with Alloy buffs, and one planet that makes Science and is stacked with Science buffs, than to make two planets that each produce a balanced output of Alloys and Science.