Topsoil is often being purchased to supplement or improve the landscape around the house. Be aware that with the addition of topsoil, a layer can be created that will effect how water moves through the landscaping. One example would be to put a very sandy topsoil, in a planting pit for a tree that is surrounded by a high clay soil with a lot of surface water. The natural tendency would be to create an area that may well be too wet for the plant's roots since the water will tend to accumulate in the clay-like soil and not drain back out.
Below is a diagram to show two different soil types in a tree planting situation
This is the basis for the recommendation to blend topsoil with the subsoil when putting down topsoil for lawn establishment. Without doing this, a plan can be established where the roots will not penetrate well and the water may not soak in. In some cases, this differential may cause erosion of the topsoil on steeper slopes.