In a general sense, why are things like dust and fog necessary? Why do we put fog everywhere? Why is there dust everywhere?

Well, I'm looking outside and every scene is coated in fog and rain outside. So, no, it's an art direction thing. A lot of art directors enjoy being able to create atmospheric environments. And I don't mean literally atmospheric, because obviously we're making a space game, but adding things like fog and dust allows scenes to be lit more volumetrically as well. So it's really just serving the needs of the art direction, I guess.