How does Dual Wield work?
Your right hand weapon swings as normal and damages at 75% of normal. Your left hand weapon receives a speed bonus of 20% but also does damage at 50% of normal.
How does investing points in Dual Wield improve this?
As with all skills, Dual Wield has diminishing returns. Each point you place in Dual Wield reduces the damage penalty imposed by using two weapons. As an example, the first point of Dual Wield causes your secondary attack to jump from 50% to 75%, the next point from 75% to 84% and so on, never reaching 100%. The primary hand increases in damage in the same way.
What is the benefit of using Dual Wield?
It allows you to use two different items with two different types of bonuses, making it easier to defeat monsters as they gain more and more resistances.