You are comparing apples and oranges to trees and sunlight! ALL COMPLETELY different things!
Why do jobs and nations exist, when planets and galaxies exist?
So what does one have to do with the other?
They are labels we apply to describe a thing.
A job, is a verb . . . It an action or task performed in some way. The description of the job is merely the attempt to compartmentalize a group of tasks that fall under the actions one is responsible for within this job role.
A nation is merely the label we apply to describe a Place so it is a noun . . . Now whether you are considering all that goes into the nation . . It's laws, trade, exports, or culture . . . It's irrelevant . . .
Planets and Galaxies are also labels we apply to describe more nouns . . . Planets are the heavenly bodies that are contained within a certain galaxy.
Your question is ill formed . . .
You could literally say "why does biting and fleas exist when dogs and breeds exist?" and have it be just as messed up? Unless your real question is why does EVERYTHING exist . . . But that would be much better to ask then the way you did.
Variety.
Because civilization exists. It doesn't matter where you are, if you want an orderly civilization and not anarchy, there needs to be established rules and guidelines and people working within them keep order and for the people to thrive.