Fascias, soffits and guttering are all finishing touches to a roof of a house but each performs a function.
Fascias: The fascia sits where the guttering is attached to the roof takes and takes on the brunt of the weather and shields your home's interior and roof from weather damage by functioning as a vertical barrier between the outside and the edge of your roof.
Soffits: The exposed area of a roof below the overhanging portion is called the soffit. Looking up at your house without a soffit would reveal exposed rafters, which is not aesthetically pleasing nor functional. Your roof's stability would be threatened by rotted rafters, necessitating significant additional financial support from you.
Guttering: Guttering helps drain water from the roof. Without it, rainwater would collect on your roof causing rot, mould and deteriorating the materials.