[narwan]

I use retreats a lot for another purpose, to misdirect the enemy artillery. As a fencer, I know the value of feints and misdirections in a confrontation!

By making your opponent think you have troops at a certain location or have a certain vector of advance you can entice him to plot his artillery there. By clearing the area and redeploying at exactly the right moment you can not only avoid his actual arty strike, but also prevent that artillery being used elsewhere.
If you attack in force at another point right after his artillery strikes the opponent might get confused as to whether this attack is the feint (to draw away the arty from it's current target) or whether the original advance was the feint... always keep them guessing!

Narwan