If the DNA from a crime scene is damaged, why would it be helpful to replicate it by PCR (polymerase chain reaction)? Even if we get billions of those copies, it is still incomplete, isn't it? How can the PCR make the forensic process easier?
This process helps with the signal to noise ratio. In theory, you could use the fingerprinting techniques to dice up the few billion copies of the DNA and carefully measure them. However, it is far easier to replicate the DNA to increase the number of molecules of DNA available to the process.
Remember, fingerprinting for crime investigations doesn't involve reading the DNA, but merely uniquely identifying it.