Building on the natural contours of the land means less impact on the site as excavation is kept to a minimum. This also eliminates the need for ugly and expensive retaining walls. Most retaining walls also involve ongoing maintenance problems.
An elevated home is definitely cooler, as building up off the ground allows for natural ventilation underneath. Elevated sites catch breezes and the open plan designs and plenty of windows allows additional cross flow ventilation and skylights in the ceiling, expel hot air.