firing more than one unit

[Midge]

The main benefit is your entire stack, if fired as a stack instead of one at a time, gets to fire at full effect before the enemy returns fire.
If units are fired one at a time other units in your stack may be Disrupted by enemy return fire before they get a chance to fire themselves !

I tend to use the "Fire by Stack" if I'm facing something "Tasty". For example in MC Danube, I fire WP armour by the stack if they're facing M1's, Leop 2's or Challengers as when the return fire comes its usually very nasty and Disrupts one or more units in the WP stack.