With regard to using flash in this situation, actually as in most any situation, it is to make printing easier and improve the photo's quality level. Think of it as doing the bulk of the burn and dodge work when the shutter drops instead of doing it "in the enlarger" and avoiding under exposure of the subject is important. He may have been stopped down a ways to keep the DOF long.

Also, we can't always wait for a sunny day, nor might we want to; a sunny day would not have the same dramatic background these shots did.