[Marek_Tucan]

Opfire prevents you from slaughtering defenseless units in your turn and having your units defenseless in an enemy turn - to balance atleast a bit the fact that game is turn-based and not real-time.
Units will opfire if:
-the enemy wanders into their field of view and/or vlose proximity
-they have shots left (experienced units might get opfire opportunities even with all shots for that turn expended)
-they have good enough morale (rule of thumb, must not be retreating or routed).

Long-range op-fire is good if you have plenty of ammo or accurate weapons, to blunt attacking force (esp. infantry with machineguns etc.)
Short range fire is good for ambushes, esp. as there is a much higher hit chance.
Opfire draws enemy fire so it is wise to cycle opfiring units and move them to prevent the God of War from descending upon them (esp. with valuable soft units as antitank guns etc.)
You can set the opfire range for given units or, with CD version, you can set up opfire filter (say you can tell your main battle tanks to fire only on vehicles with front armor over 10 points and under 100 points, and only at distance of 25 hexes or less).