Fiberglass is light, 100% fuel proof, doesn't peel up around the edges or bubble in the sun. You can paint it so your plane doesn't look like a plastic toy. You can repair it so that it never looked like it was damaged (repair, sand, fill, prime and paint instead of patch, patch, patch).

It has some strength. I didn't mean that it's weak. But the reason for using it is to give the plane an excellent base for paint.