The reason that an investigator should be knowledgeable about the legal process is that the investigator is gathering information and evidence that will be used as a part of that process. This means that it is important for the investigator to know the process so that they can know what kind of evidence is needed and how to document and preserve the evidence.
If investigators do not know the components of the legal process, they will not know what sorts of evidence will be needed to obtain a conviction or a guilty plea. If they do not know the process, they will not know how to document the evidence to ensure that it will be admissible in court. Investigators will be much more likely to make mistakes that end up being fatal to the case when it is brought to court.
Because an investigator’s job is to collect evidence to be used in court to convict a defendant, it is important that they know how the process of criminal justice works.