The concept of chain of custody is based on the idea that it is important to be sure that a piece of evidence against a defendant in a trial could not have in some way been altered to make that defendant look more guilty than they really are.
In trials, there is typically a variety of physical evidence. In some cases, the evidence does not look particularly distinctive. It could easily be confused with other similar things. For example, let us imagine that the police discovered needles and cotton balls that were allegedly used in illegal injections in a defendant’s home. Those are fairly generic things. It is therefore necessary to, at the time of the trial, prove that these needles and cotton balls were the same ones taken from the defendant’s home and that the evidence could not have been tampered with in the time since it was taken from the home.
In order to do this, it is necessary to establish the chain of custody. Whoever took the evidence from the home must be able to establish that they took care of it properly and must be able to say what they did with the evidence. Whoever got it next must, for example, testify that they kept it safe from tampering until the time of the trial.
This idea is important because it ensures that evidence used against a defendant will be valid evidence that has not been tampered with.