Well ... I guess if your building needs a roof ...
DUH!
How does a roofing contractor help in Construction?
They apply shingles, tile or metal roofing on buildings.
If you want the building to be dried in, you need a roof. And you want it dried in before you put in the electrical system and the walls.
Do you want to repair your roof on your own? If not then roofing contractors come in handy. I myself prefer to rely on professionals and that's why when I needed roof repair I decided to find a roofing contractor in Carmel. And I managed to find a decent specialist. You can read here if you need such services too.