Class Mods are character-exclusive upgrades, that when equipped, offer various stat buffs and powers. However, what truly makes them exciting is the additional skill points they provide.

There are two key benefits this enables. First off, Class Mods allow you to exceed the normal point cap for each ability on the skill tree. Though more importantly, it also lets you bypass restrictions and obtain certain skills without having to spec towards it.
As you can probably imagine, this unlocks a tremendous amount of build diversity, as you're free to spend your skill points as you like while reaping the rewards within uninvested territory.