It is necessary whenever the vehicle cannot continue and cannot be made to continue safely. Major mechanical failure is the obvious case: engine or gearbox trouble, a clutch that has gone, a snapped belt, a cooling system failure where continuing would destroy the engine. Accident damage is another, even minor damage, if a wheel is out of line or fluid is leaking. It is also necessary when the fault itself is small but the circumstances are not, such as a puncture with no spare, or a flat battery on a stretch of road where there is nowhere safe to work. Location often decides it on its own. On the A12, the A406, the A13 or the M11, recovery is the right answer almost every time, because working on a live carriageway is not worth the risk to anyone. If you are in a live lane, on a blind bend or on a fast slip road, call immediately rather than assessing the fault yourself.