The Negative Feedback Amplifier allowed long distance coaxial cable transmissions to be amplified without distortion. Harold Black's invention made possible the growth of the network and for undersea cables to work. When you have transmissions over such distances, you need a number of amplifiers (in repeaters) to boost the signal. The NFB amp reduced distortion in this multiply-amplified signal.
The concepts involved in using negative feedback have broad applications well beyond electronics.
Black conceived of a negative feedback loop on a signal during a rare "flash of insight," famously, which happened when he was on his morning commute to Bell Labs in NYC via ferry. In this film, Black describes the event. For a much more detailed picture of the problems Black was already working on that led him to this conclusion, a long Oral History by Black is at IEEE.
Black continued to work at Bell Labs through WWII and up until 1963. He was awarded 66 U.S. Patents and published 42 papers and 2 books in his lifetime. Black passed away in 1983, three years after this film was made.
Footage Courtesy of AT&T Archives and History Center, Warren, NJ