Increse chance to land a debuff.
Godd for Cranes, tanks

As songogu said, success rate increases the chances of landing a debuff on your opponent. For example: Stuns, sleep, slow, confusion and others.

Success rate is good for classes that require you to land debuffs - SKs, karmas, turtles, cranes all need a high success rate necklace as they have many debuffs.