Forensic science is defined as the application of science to the law. In criminal cases, Crime Scene Investigators and Forensic Scientists are often involved in the search for and the examination of physical evidence which might be useful to implicate or exclude a subject suspected to be involved in a crime. Such evidence commonly include, among others, blood and other body fluids, hairs, fibers from clothing etc, materials used in buildings such as paint and glass, footwear, tool and tire marks. Proper examination of the evidence and the reconstruction of the crime scene is vital in both the prosecution and the defense of the suspect(s) involved.
From a defense standpoint, the evidence must be reviewed by an analyst to determine if the findings coincide with that of the prosecution. I've often been told something such as "they found a fingerprint and it matched my client". When I asked how they were certain, I've heard replies like "because they said it matched". A second opinion by an outside source can assist the attorney in determining the best course of action for their client. That opinion may differ from the findings of the person who initially collected and/or analyzed the evidence as much is based on individual perspective. An independent analysis could also assist to confirm or refute the statements of witnesses and suspects, as well as the facts of the case.
It's up to everyone to insure that those who are charged with a crime receive a fair trial. Just as much emphasis needs to be put on their defense as it is their prosecution.