There are several different kinds of heat transfer: conduction, convection, radiation, and advection. The one that most affects you and your attic, however, is radiation.
Heat, in the form of ultraviolet radiation from the sun, in the summertime can heat your roof up to 150 degrees. This heat has to go somewhere. It does, of course, radiate, conduct, and convect downward into your attic, if unfinished, which then moves through the ceiling into your living space. Even if the heat that enters your living space via the attic has lost 70 degrees, it could still be hotter than comfort level, and cause your cooling demand to rise.
The radiation part of this process can be greatly reduced by a simple aluminum foil barrier — Alum-A-Cover. By introducing this barrier, which can deflect 90% of the radiation heat from the sun, you can greatly reduce the temperatures of the attic space and living space.