I thought the same thing. Judging by the presumably clear lights on the bottom of the light bar maybe it’s like a “mobile booking station” or some kind of “on scene HQ”???
Here it is with the lights folded up, from what I’ve read they would raise them up at an accident seen to warn the public about the accident
BPD & MDC Police on site of a runaway Penn Central train accident that blocked most of the Northbound lane of the Southeast Expressway in South Boston August 21, 1969