Regular aircraft are required to have enough fuel to be able to divert to the alternate airfield in their flight plan plus all the margins for taxiing and waiting for a landing clearance. If a plane is low on fuel they can declare an emergency and get priority over other traffic, but that obviously is disruptive and should not happen on well planned flights unless something pretty significant happens after takeoff.
> and should not happen on well planned flights
You make it sound like it is acceptable for a badly planned flight to have a fuel emergency. It is not.
If there is a fuel emergency (or the plane lands with less than 30min of fuel) there will be an incident investigation that treats the situation as serious as if the plane had crashed. If the investigation discovers it was nothing more than bad planning, (at minimum) the planning procedures will be changed to ensure it never happens again.