Lower Resist is highly effective in a multiplayer setting where you are helping your allies' weapons and spells do more damage. This also helps the summonable creatures that use elemental attacks, such as the Skeletal Mages or the Fire Golem, not to mention any Revived monster you may control.
Lower Resist lowers Poison, Fire, Lightning, and Cold Resistances. It does not lower Physical or Magical Resistance.
Monsters Resistances can be lowered into negatives. When a monster with 0 Lightning Resistance is cursed with level 20 Lower Resist (-62 to All Resistances), the monster will now have -62 Lightning Resistance. The damage taken from Lightning attacks would be increased by 62%.
Lower Resist can remove elemental and poison immunities. When breaking an immunity, it will function at 1/5 effectiveness.