This is a complete shot in the dark, but I think it lets you put much larger injectors into your car. Look at your intake manifold area, the piece of metal that the injectors plug into is the fuel rail. The JWT one is probably manufactured to fit a different kind of injector that is more common than the stock injectors.
U see the nissan injectors that the SE-R uses are Galley Fed (they feed from the sides). They work great but the cost $$$$$ if you want bigger injectors. You can use a top feed stlye injector in your car with that kind of fuel rail. "Do I have to buy JWT's?" only if you want to spend that much for a fuel rail.
The JWT rail is also bored out on the inside to 1/2" compaired to the stock that is 1/8"? This allows for less chance of fule starvation under high boost @ WOT. So unless your running turbo you really don't need this mod.
[This message has been edited by Mike Gosselin (edited 02-14-2001).]