You cannot carry fuel for every possible scenario!
Standard fuel policy of many major airlines includes ALT fuel on every flight. This is for many reasons including the closure of a single runway airfield.
It has always puzzled me as to why QF doesn't carry this fuel.
Once a QF flight passes its DPA (assuming a happy QF Capt who carries flight plan fuel only) then he has no fuel available for diversion.
Imagine the pucker factor when he is told by the tower to go around due to prior landing aircraft being disabled on the only available runway...