Piggy back chip fools the ECU into thinking that certain things are happening that aren't, and then passes on information to other parts that the ECU isn't sending!
The chip has also been designed with one specific set of mods in mind, so unless you're running EXACTLY what it was designed for then you never know if it's running 100% or not.
By getting a custom remap on either a standard ECU or a replacement one you'll know that what's being done is tailored for your car. You get the most performance, reliability and safety. It's the best of everything!