These are used when squeezing more hp from an engine. Also certain drag classes require them due to the stock one potentially coming apart. They don't make more hp. The damper is fluid based and self contained where a factory damper is rubber bonded to steel which is where failure can occur.
Went Fluidampr myself, piece of mind as everyone else states, 200k km on the original so while I am in there she can come out ATI I have on other vehicles and would go with either company again.