harrasing fire can be VERY useful to disrupt enemy truck-borne infantry advance in "safe zone"
EDIT: Mortars are good "first responders" and while their range is limited, it can be a blessing as you do not waste them in supporting other units.
Generally, I treat 81mm's as "Co commander's pocket artillery" (or Bn) and heavy arty as heavy hitters.
Pretty nifty thing mostly in WWII are infantry guns.