The obvious reason is to seperate the bosses from the troops. This can be good and bad. It makes it easy to identify who's in charge at a chaotic scene, even for civilians. On the other hand it visually establishes an "us vs. them" dynamic among the police employees themselves. It kind of all comes down to what the police adminstrator who makes the decision values as a "leadership" ideal. Do you have your supervisors stand apart from the personnel they lead to inspire and command or do you visually state that "we're all in this together" and put everyone in the same uniform?
We only had one chief who put management in white shirts. When he left, the next guy switched them back.
I was always a contrarian and I preferred the bosses wearing white shirts. The "us vs. them" dynamic between management and labor always exists anyway. Might as well make it hard for them to sneak up on us when we're grumbling about their stupid decisions.
That is a really good explanation, thanks!