In general, creating division between classes, races, religious groups presents opportunities for individuals to gather power by taking up the cause for one group. By establishing an “other” you have an enemy to rally support for your leadership. This is the basis of power for every demagogue from the Cleon in Athens to Joe McCarthy and the American Red Scare. This is also a core theme in Orwell’s 1984.

In the context of the OPs topic, there are those who honestly are trying to improve the world, but many people have new found careers and political power created by the division on both the Right and Left.

The "who" is pretty easy - it's corporate media outlets: https://www.tabletmag.com/sections/news/articles/media-great...