It was a world war because it involved all the worlds superpowers and was fought throughout the world eg. Western and Eastern European theater, North African theater, Pacific theater etc.
World War II managed to advance world technology, introducing better planes, neculear power and weapons, and advanced medical sciences. No matter how horrid the war is, science always grows out of it...
At the start of World War II in 1939 it was limited to just a European war, with the British & French calling it a "phoney war". It wasn't until the wars of Asia & Europe joined up it was...