Public transportion, shorter distances to wherever people need to get to go, roadways used by more traffic, transportion of food and goods to a centralized area...
A crowded block sharing electric, phone, fiber optic lines, gas, water and sewage pipes, the materials for making sidewalks and streets, garbage pickup, street cleaning, plowing during winter, mail.... and more add up to more efficient use and thus less waste than the same in places where the nearest neighbor is a mile away.