Well ...
How important or not is subjective to the user , really .
For F2x series, there's no gauges to know if your car is warmed up or not unlike F3x series where there's at least an oil temp gauge.
So now that I've a P3 gauge that can read my coolant temp ... I would know that I can actually start to rev my car once it reads 80deg rather than "butt feel" .And if it's reads a few degrees above the norm, I would know something could be wrong and send it straight to the workshop.
If there's no gauge, I would have to wait till the engine check-light comes on but by then, the low coolant levels might have already affected other parts of your car.
There are also many other readings which u can view too. (Intake temp, throttle response, speedometer, rpm, etc)
As mentioned, I love the 0-100 function. Pretty cool to actually know your actual timing!