To me he refined the lighting of photography, and was able to get a very distinctive level of communication going with his sitters. It's a very rare quality I see in most photography. I don't even know who a lot of the people that he photographed are; for all I know they could be strangers. Does it really matter that much that they are celebrities?
In addition, the portraits are beautifully lit for an appearance suiting the sitter. They are beautifully crafted prints with tons of visual impact. I mean, look at those prints!
Finally I have a lot of respect for how he returned the favor of being taught, by being a teacher, offering apprenticeships to young photographers.