The fuel rail is a pipe that delivers fuel to the injectors of your engine and is required to operate your engine. If you have a high performance race car and have added the Weapon R fuel pressure regulator, then this part is a must. Their fuel rail adapter can handle more flow than your OEM one and it will allow your engine to perform better. Weapon R makes them specific for your engine as well so installation and fit is just like factory.
You can think of a fuel rail adapter as a bigger straw than the smaller one you were using. Weapon R has designed theirs to handle more flow, especially from the fuel pressure regulator. If you have an OEM fuel rail with an aftermarket fuel pressure regulator, then this setup would not be of any benefit because it's like forcing an apple through a hose and just does not work. The bigger Weapon R fuel rail can handle fuel at an extremely fast rate and is a perfect fit for their FPR.
All aftermarket fuel rails are not the same and some may not even fit, even if they are a millimeter off. Fitment is so crucial of this part because if there are any gaps, fuel will leak out and cause major problems. Weapon R produces their identical to the factory one so there are never any problems.
The Weapon R one is also much more appealing than your factory part and immediately hits the eye when you open the hood so now you have the best of both worlds which is performance and style.