Final Reserve Fuel being the minimum amount of fuel with which pilots should plan to land.
Crumbs, what's hard to understand about that? You are not allowed to
plan a flight that would take you below this figure at landing.
Regardless of the endurance you actually achieve during the flight itself (it might be more due tailwind or lots of 60Kt orbits, or vv) FRF is that by which you MUST land, and further, when it becomes apparent that you will land with less then a MAYDAY is required. You might well decide to fly another 20 mins on task with base 25 minutes fuelaway and not make a MAYDAY, as long as you land (wherever) at/by FRF.
One aspect of it is a pre-flight planning restriction to prevent chancers from trying to stretch range "because we'll probably find a tailwind/we never burn that much anyway", the other is to make you land when you reach that figure regardless of what you planned.