The Iowa Life Changing website has a quick tour of Green Roofs currently seen in various areas of Iowa. Green roofs are catching on quickly and have several advantages over traditional methods.

Social Benefits
Improved livability.
Improved physical and mental health from exposure to nature.
Increased sense of pride and place, levels of trust, civic participation, less violence, aggression, vandalism, and littering.

Aesthetic Benefits
Green roofs are much better looking than asphalt, gravel or tar.
Natural views create more productive, healthy, happy, creative, relaxed people.
Green roofs expand the usefulness of buildings via patios, gardens and vistas.

Urban Heat Island Mitigation
The urban heat island effect is the temperature difference between urban areas and their rural surroundings.  The temperature differential causes air currents and dust, and even contributes to violent weather events within urban settings.
Green roof vegetation helps by cooling the air, slowing air movement and acting as a substrate for pollution to settle out and detoxify.

Noise Reduction
Plants, soil, and air trapped in the soil are great acoustic insulators.  Tests indicate that green roofs can reduce indoor sound by as much as 40 decibels, which is of great benefit to occupants of buildings affected by airports, industry, trains and traffic.

Fire Prevention
Succulent green roof plants help reduce the risk of fire.

Water Conservation/Reduction of Storm Water Runoff
The soil in green roof systems acts like a sponge and absorbs excess rain water.  Research has shown that extensive green roof systems can reduce runoff by up to 90+% annually (varies with climate, soil and pitch of roof).
What runoff remains will usually occur hours after peak flows, providing additional time for sewer systems to handle the runoff burden from impervious surfaces.

Habitat Recreation
Plants and soil help recreate habitat for butterflies, insects, and songbirds.