Why do we have rules that govern the admissibility of expert witnesses, forensic evidence and scientific testing?

Why do we have rules that govern the admissibility of expert witnesses, forensic evidence and scientific testing? Do you believe that the current rules or court tests provide adequate safeguards for this process? What public policy issues or considerations do you think might be behind these rules or tests? How do these rules and court tests influence the gathering of crime scene evidence or investigation of a crime scene?