It gives you a real world figure that lets you see what the fueling is doing without blindly stabbing with guesses.
If you are introducing an adjustable fuel regulator into the mix that adds another variable. For example, if the car is overfueling, how far do you dial the FPR back? It's quite an important question as if you lean out the mixture too much the engine runs lean and detonates causing internal damage.
The concern is that we are assuming it is overfueling but there could be an ignition/wiring/ecu fault at work causing unburnt fuel and poor running. It's very difficult to give a definite answer over the net. Last time I had a similar issue it was a dead coolant temp sender so you could be dealing with a random sensor fault.
If you have a local tuner they should be able to check air/fuel ratio and offer some advice. If nothing else do some reading on AFR values, might help you confirm the symptoms.
