In our legal system, evidence is used to convict or to exonerate people charged with crimes. Evidence can, for this reason, have a tremendous impact on the life of a person charged with a crime. Since it can have such an impact, it is vital that it should be preserved intact and not contaminated so that the defendant is not convicted on the basis of faulty evidence.
For example, let us imagine that a man is being accused of murder. The police impound the clothing he was wearing when he was arrested. The man claims that he was never anywhere near the scene of the murder. Let us then imagine that we are in court and the prosecution introduces the shirt the man was wearing. It has blood stains on it that can be matched through DNA testing with the blood of the victim. This would be very damning evidence.
This shows us why the evidence needs to be preserved in its original state. The shirt needs to be kept just as it was when the police arrested the man. The police, for example, must not contaminate the evidence by placing the victim’s blood on the shirt. That would clearly make it more likely that they would get a conviction, but it would be obtained under false pretenses.
If evidence is not preserved in its original state, it has little value because it would be plausible that it could have been altered to help or harm the case in which it is being presented.
The reason it is so important to preserve evidence in its original state is due to the adjudication process. Evidence that is "excluded" may be the only link to justice for the victim, victim's family or defendant if innocent.
Our system of justice must be preserved and uncontaminated for it to be applied fairly.