CEM overhauls under british rail relied on transplanting components such as bogies, engines, gensets, cabs etc with low hours removed from withdrawn examples to keep others in traffic. The various lines of withdrawn and unservicable locos in willesden, Toton, Carnforth, Crewe, Barrow Hill, Derby etc testament to the fact this practice still continues to this day. It is no different to you turning up at a car breakers yard looking for parts to salvage for your own vehicle. Such parts are far cheaper than going to the vehicle dealer or a motor factors and could also yield parts no longer available.