There used to be a time when damage types were part of an element wheel, whereby baddies were weak vs. some and strong (in some cases, immune) vs. others. This concept wasn't restricted to just fantasy dungeon games or "train wildlife to become pit fighters" games. For example, Freelancer. You had shields that were effective against some energy types, while weak against others, and middling against everything else (Ex. Positron Shields, Good vs Tachyon, Poor vs Lasers).
The other half of this concept, the one that Cryptic/PWE got right, is that you almost never had to worry about being unprepared, since groups of baddies were almost always homogenous, and thus always dealt the same damage type (trolls were rarely found in the company of ice elves, flappy birds and bugs were never in the same place as fish monsters, and pirates never fought side-by-side with bounty hunters).