Why is the setting in Fahrenheit 451 so important?
Because of its deep themes of censorship and oppression, it is fitting that Fahrenheit 451 takes place in a modern, future city. Cities are often thought of as hives, stacking people upwards with less focus on quality of life and more focus on keeping the city functioning; because of the book ban, the people in the city are not individuals, but members of a collective.
"...they all say the same things and nobody says anything different from anyone else. And most of the time in the cafes they have the jokeboxes on and the same jokes most of the time..."
(Bradbury, Fahrenheit 451, Google Books)
The government controls the news and entertainment through television, and the people take it in and become defined by it. Just as a city is a homogeneous structure, largely composed of similar buildings that serve similar purposes, the people inside it are all the same. The city also serve to showcase the alienation that bothers Montag; even though people live so close together, they have almost no interpersonal relationships, instead treating the false people on their television screens as a surrogate family.