It is necessary whenever waiting until tomorrow costs more than the delivery does. The classic case is a car stripped and up on a ramp with the customer expecting it back that evening, and the one part needed sitting at a supplier eight miles away with no van available until the next round. It also comes up when the wrong part has been sent and both the return and the replacement need moving in the same run. Heavy and awkward items are a frequent trigger, because standard couriers will either refuse them or take three days: engines, gearboxes, axles, exhaust systems, doors, bumpers, windscreens and full sets of tyres. Weekends and evenings are another, since most supplier delivery rounds stop mid-afternoon on a Friday. And it is necessary any time a main dealer has the only part in the region and will not deliver it, so someone has to physically go and collect.