Fiberglass hood scoop

[cowboy90]

Does a fiberglass hood scoop play any role when put on a steel hood or is it just a decoration?

[Ghoste]

It doesn't matter much what the scoop is made of, the functionality of it depends on it's size, location, and shape and whether or not there is a corresponding hole cut into the hood to allow it to funnel air into the carb(s).
Fiberglass is a common scoop material because it's cheap, easy to work, and lighter. 

[Silver R/T]

you need to know how to attach it on there. Fiberglass and metal do not bond pretty good if you don't know this already. Some people try riveting them on there and then glue the seam and bondo over that...it might last a while but eventually it might crack.

[Ghoste]

And just as a point of information, the Ramcharger style scoops like you would find on a 69 Coronet RT, would be functional and fiberglass whereas the scoop on something like a 71 Duster Twister looks similar and is fiberglass but it is just for decoration.  So the real answer to your question is... both.