Sooo... Let’s see. If the USPS murdered someone or was actually doing B&E and stealing stuff, that’s all for the IG and the locals can’t do a thing?
Who has jurisdiction depends on what crime has been committed. Like when a kidnapper takes someone across state lines, it goes from state/local to federal. If you rob your neighbor, it’s state/local... if you rob a bank, it’s federal.
If you see someone getting murdered or a crime occurring in real time, duh, you call the police who can be there in less than 5 minutes. If THEY find out the crime was committed under specific circumstances that may call for federal intervention, they decide amongst themselves who has jurisdiction to prosecute.
If you have suspicions that a postal worker MAY be doing something wrong, the inspector general can have someone investigate that postal worker specifically. I know of specific postal workers who have been followed and video taped breaking the law by inspectors inside the postal system.
The local police do not have time to sit around and wait for a postal worker to do something wrong on their time.