It helps when you are fighting someone with an AC less than 0.
When you take a swing/shot at someone, the computer makes the following calculation...
attackers THAC0 - defenders AC = lowest needed attack roll on 1d20.
So, if you have a THAC0 of 0, and the defender has an AC of 3, you're going to hit them with out any problem. However, if the defender's AC is under 0, then you risk a small chance of missing. A THAC0 of -9 would probably hit any enemy in the game who isn't protected by some sort of magic or has some other modifiers.
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