The evidences of DNA is very important because it helps us to know the criminal. Even if the there is any physical evidence left like left over hair, blood, body fluid or finger print.
These helps the forensic science to determine the criminal of the crime. The blood sample or the left over evidences can be used to do the DNA fingerprinting and then matched to the file of the criminal record or the suspect of the crime.
If the matching takes place then it is considered that the person is criminal.