When navigating the trailer through tight clearance issues you gain an advantage if the back of the trailer’s profile is reduced. By designing the back of the trailer to be narrower than the rest of the trailer, the trailer's silhouette become smaller as you pass through constricted areas. Because of the length and size of a tractor drawn aerial, deciding not to add compartments to the end of the trailer can pay dividends when the trailer slides through locations that would otherwise be too tight.
These three examples demonstrate the difference between apparatus that have compartments added to the end of the trailer compared with trailers that do not have those compartments. (Click to enlarge pictures)