There are 3 major reasons to install a fuel cooler on your tuned PD TD:

Reduced intake diesel temperatures will reduce temperatures inside the motor and consequently EGT temperatures.
There’s a fuel limiting table in the code that limits the injected diesel quantity, depending on the fuel temperature. To keep injection quantity at its maximum you need to keep the fuel under 80ºC. This may sound simple, but under spirited driving or track day situations you’ll easily get fuel temperatures in excess of the 80ºC. During the summer months, I easily see in excess of 90ºC.
Diesel at higher temperatures has less lubrification properties, when compared with cooler diesel.