The roof scoop is only used on the WRC cars for subaru and other rally racing cars. It cools the cabin because they don't have AC or anything in the rally cars to save weight. The regular street cars do not come roof sccops. We do have hood scoops though, and that is to cool down our intercooler which keeps the turbo and air cooler for max HP.
The Mustang probably has it because they removed the AC for weight saving, but stupidly put in a sound system for show. It looks like the car is meant for some kind of competition though, so it is there for looks (if it is a trailer queen) or function (if they race it all).