i always assumed it was so the factory (and the neighbors and roads) weren't covered in smoke
Of course by the 3rd of 4th generation it becomes a distinction without a difference. But understanding patterns of development is important. If today you want to prevent poor people from tomorrow living in polluted areas, rich people have to make it easier to build affordably in nicer areas--e.g. allow increasingly dense development so poor people don't get pushed toward industry.
There is a long history of this sort of thing.
A video about this: https://youtu.be/3VJT2JeDCyw?si=lzbBWQjXk-o0cblD