Alarm and Response Regulations allow you to bundle up a lot of vehicles into larger units, to help you manage more complex missions, which require a lot of different vehicles and to save time with the vehicle selection in the dispatch screen (i.e. you want to dispatch units to a mission with around 20 or so vehicles, instead of selecting all 20 vehicles individually, you can create ARRs to speed up that selection process.
While ARRs (Alarm and Response Regulations) aren't tied to specific vehicles but to vehicle types, vehicle groups directly use specified vehicles.