The impact of this film, for me, comes from its depiction of the idea of justice being a concept (or set of concepts) that is not self-evident and cannot defend itself but must instead be defended. One man is asked to make this defense, against powerful and popular interests.
The conflict that he faces within himself is a reflection of the nature of justice in the film - a sense of impersonal and value-driven duty to see clearly and fairly.
I think that Redford's film carries significance in a couple of ways. The first would be that the production company that developed it, the American Film Company, is animated with a rather different approach in making films:
[The American Film Company believes that] real life is often more compelling than fiction, [and] produces feature films about true stories from America's past.
This would be significant in that the company seems to have made a financial commitment to historical films, something that is not as openly embraced in mainstream film production today. Another reason why the film is significant is that it centers on a topic that has been largely overlooked in Hollywood films. The study of Lincoln's assassination has not received its share of Hollywood films. Contrary to Kennedy's assassination or other political backgrounds, the death of Lincoln has not been something that has served as the center of many Hollywood films. Redford's film is unique in that it focuses upon both the assassination of the President as well as the trial of Surratt, and begs the question of whether or not justice was carried out in the trial of the murder of one of America's most important Presidents. I would say that these help to ensure that the film is relevant and profound in its impact.