Archival footage (sometimes referred to as stock footage) can be used as a way for studios and filmmakers to save money by using something that's already been filmed before. This has been notoriously done in b-movies and animated movies as a way for filling up time. However, it can also be used creatively. Some filmmakers use archival footage to make a collage-effect for their film similar to what many post-structualist novelists do in text.
This clip from House uses archival footage in the postmodern and the commentary sense in the way that it juxtaposes the marriage scene with the footage from the atom bomb. it equates the two and also shows what the death of the aunt's lover came rom without having to say so directly.