The OE FPR is a rising rate FPR but you cannot set the base pressure. An aftermarket FPR will allow you to set a higher base pressure. What this allows you to do is spray more fuel across the range. This means you have effectively more fuel from the same injector so you increase your fuel pressure at the FPR, scale back your whole range and this means you have the same flow across the board but you have a little more head room at the top. There are also rising rate FPR's which for each pound of reference (boost) pressure they add x (4,8,10,12, etc) pounds of fuel pressure. You need to be careful in selecting an FPR.
you need an FPR when your injectors are apporaching max reliable duty cycle (85%) and you want to squeeze out a little more performance but you don't need so much (or don't want to spend so much) to go to bigger injectors.
Good example is a stock set of 330's running 3xx WHP - you'll be pushing thier limits. But if you're running about as much as you want to run for power, then don't spend 800 on your fuel system when a $200 FPR will get you over the hump with less expense and work.
Check over at clubna-t for more info on fuel systems and FPR's.