I thought the shorter wheelbase trucks with short trailers were used for local deliveries in the city. Space is usually at a premium in cities and the shorter version trucks were able to back into cramped loading docks much easier. At least that's what trucker friends have told me.
You'll find some interesting pictures of L700's at www.sweptline.com , including L700 fire trucks.
Note: these were MEDIUM duty trucks with gas or Perkins diesel engines.