Why is Sydney Carton's presence at the court of great importance?

The presence of Sydney Carton at the trial of Charles Darnay is important, not from a “legal” standpoint, but to set up the physical resemblance between the two characters as significant. Darnay is released when Carton points out to Stryver that he and the prisoner look alike, thus throwing the possibility of doubt on Barsad’s testimony that he recognized Darnay. This becomes extremely important at the end, when Carton switches places with Darnay in prison, taking his place at the guillotine. Without the similarity that the two have, this ruse would be unlikely to succeed. Thus Carton can fulfill his promise to Lucie that he would do anything to save her or anyone she loved. As the book begins, so it ends: Carton manages to save Darnay’s life, but this time by sacrificing his own. Carton becomes an important part of Charles and Lucie’s life even more after his death.