Alaska Senator Ted Stevens was prosecuted for corruption in office by the United States Attorney's Office. He allegedly failed to disclose gifts made to him. He was found guilty following a jury trial. The jury based their decision on what was provided to Stevens' defense team. However, what Stevens and the jury didn't know was that important statements of witnesses and testimony which was exculpatory (tending to show Stevens' wasn't guilty) was not given to the defense.
The lawyers from the US Attorney's Office knew that their most important witness was having sex with underage girls and part of this witness' testimony against Stevens was most probably fabricated. Not only did the prosecutors know about these issues with their key witness, which could be used to impeach his credibility, but they covered it up. Not only didn't they turn the information over to the defense, they hid it from Stevens' defense team. Apparently, the prosecutors' desire to obtain a conviction overcame their sense of right and wrong, justice, and ethical duties.
Why is this important? First of all, the defendant's right to a fair trial hinges on the prosecutor being fair and open with all evidence which could be termed exculpatory. Anything that reflects on the credibility of key witnesses must be turned over to the defense. Failure to do so cheats the system, the defendant in question, and undermines everyone's faith in the criminal justice system.
In this case, Senator Stevens was unable to continue as a US Senator. The citizens of Alaska were deprived of their elected representative. In the end, if the prosecution, who controls the flow of information and conduct of law enforcement, can't be trusted to respect justice then every prosecution of every defendant is called into question.
Respect for the law is necessary for the citizens of our country. If you don't believe you will be treated fairly in the judicial system, society can descend into anarchy. People will take the law into their own hands. Corruption of the judicial system leads to abuse and the trampling of citizens' rights. If you can't trust the prosecutor to be honest and to turn over evidence which could lead to exoneration faith is lost that there is any justice.
The criminal defense attorney can require the court to order the prosecutor to turn over exculpatory evidence. But, if the prosecutor seeks to subvert justice then each and every one of them should pay the price and find themselves in the defendant's seat at the table of justice.