It just increases damage
Because augments don't stack in effectiveness, you need two different augments with essentially the same effect. If you combine a devastation augment with a lucky charm augment then you increase your weapons damage as much as conceivably possible, because they both increase damage.
These two augments are the secret to the Red Dragon's power, actually. Otherwise it's kind of crappy. Putting these two augments on anything is a massive boost in offense.