Dim Vision reduces the sight range of enemy monsters. This might be useful as it prevents enemies from seeing you and thus attacking you. Enemy spellcasters who deal their damage from afar such as Zakarum Priests will be rendered passive, which stops them from casting spells, thus making them ripe for the taking. It will not work on other players, Superuniques or Oblivion Knights
Damning an enemy to the darkness of their own evil, this curse surrounds the afflicted in a sphere of perfect darkness and silence, blinding them. This allows a Necromancer and his party to slip by unnoticed or maneuver close for a more effective attack. Decreases radius of awareness.
↑Diablo II Manual, page 71. Blizzard Entertainment, 2000.