Why are the barriers to communication within and between organizations sometimes a good thing?
In the context of police corruption, which is where you have placed this question, compartmentalization within and between organizations can be a good thing because it can lessen the potential scope of corruption. If there are no barriers between or within organizations, it is easier for a corrupt member of the organization to do a great deal of damage.
Let us say that we are talking about a police department that has a couple of corrupt officers in the traffic enforcement department. These officers are taking bribes to fix tickets and do minor things like that. If the department is compartmentalized, that is all the harm they can do. If, by contrast, they have access to every aspect of the department, they can do much more harm. They might, for example, be able to access the files in an important drug case and tell the suspects things like what evidence the police have and where they have surveillance. They might even be able to destroy files that are of help to the police in their investigation.
Therefore, compartmentalization can be a good thing. It can prevent corrupt officers from doing as much damage as they would be able to if there were no barriers.