What is nitrogen needed for?
Assuming a biological reference: A large amount of nitrogen will be in proteins. It is also found in nucleic acids that make up DNA and RNA. The head groups of lipids also have nitrogen in them. There are of course more biological roles (nitric oxide is a signaling molecule). Other uses for nitrogen are cooling applications (liquid nitrogen), rocket fuel (maneuvering rockets, the large ones are solid fuel or liquid hydrogen/liquid oxygen), fertilizer, explosives, provide an inert atmosphere for chemical reactions, and many others.