What are the benefits of having a fibreglass body on a car?

There really isn't any "real" completely fiberglass car bodies anymore. Hasn't been for many years. They are all a composite material mixed with fiberglass. They really aren't much lighter than all metal cars of today. They use them now mainly on high powered cars because of the flexibility of the material to take the stress of speed and power. 

After working on many over the years, I prefer metal or the plastic composites as they don't get stress cracks and are much easier to repair.