Grass obtains three of the four essential factors (air, water, and nutrients) from the soil, but many soils are less than ideal for growing grass.
Most of our housing developments have very little top soil. When basements are dug, this dirt is placed on top of whatever good dirt was there. This is commonly referred to as "basement topsoil." To further compound the problem, a lot of equipment runs over the ground compacting the basement topsoil. As the soil particles are pushed together, air spaces are removed. This reduces the water holding ability and makes it harder for the roots to penetrate.
The value of proper site preparation and soil improvements, before any planting takes place, is that it will be easier for the grass roots to penetrate deeply and evenly. Deep roots will make the lawn more drought resistant and will allow for more efficient water and nutrient use from sub soil.