Plea bargains help to unclog the court system. If the prosecution has a "weak" bundle of evidence, this can give assurance that some form of punishment will occur. The negative is that a charge of favoritism might be made by the victims family an so they are often brought into the discussion for agreement sake. Cost savings are great because the cost of a trial is very expensive
The practice of plea bargaining does have positives and negatives. On the positive side, it does help speed up the justice system and probably saves money. On the negative side, victims of crimes may get upset when a perpetrator gets a light sentence from a plea deal.