I have a 1998 RS - it has the singular dubious distinction of NOT having a fuel gauge warning light.
I am limited to watching that needle drop till it is "on the floor" and so far, I have yet to run out of fuel - but I digress....
I believe most cars are fuel warning systems are designed so that when fuel is down to the last (approximately) 20% capacity (in the case of our MX-3's) the light comes on. Now this gives you enough fuel reserve to make a bee-line for the nearest Gas (Petrol) Station and make it with enough to get to yet another if that one is off-line.
The large reserve comes into its own if you were on a motorway, and the next Gas Station was 50 Miles from your location. I presume you understand the logic to this.
It doesnt matter much when you have 5 or 6 Stations dotted around, within 3 miles of your home or work location. But those who regularly find themselves in the middle of nowhere will find that large reserve and early warning capability a godsend.
Usually I end up filling up to the brim with anything from 38 Litres to 43 Litres. By the time I get to the latter, I have really been riding around with my heart in my mouth & the car metaphorically running on Gas Fumes.
Hope this helps in some small way.
Thunderstorm