They have contacts that make it possible to get the coverage you need and can insure the issues are stressed the way you want it done. The negative is that these PR people exist by having long term relationships, usually with political or business clients. Thus the cost of their expertise will be very high. Make a list of all the media whose coverage has included cases like yours. Local media is very important because it can elicit strong reactions directed at the police and prosecutor because these reactions are coming from the neighborhood of the crime.
National coverage can keep local interest alive.
Cases which appear in these venues usually occur after local coverage has ebbed, thus, this coverage can precipitate renewed coverage at the local level. These venues don't usually pay for your story but, if they do, it can taint your cause. So, be careful. If they do pay, it is only for the BIG story and for exclusive rights. Exclusive rights can be detrimental to your interest if the real issue is to get the criminal tried and convicted in a court of law because it reduces the breadth of coverage.