Writing something down/off means that you're asserting the asset has less/no economic value left, and you can basically treat the delta between its book value and the written down value as an expense for taxation purposes. But if you e.g. write off a car from its real value of $10k to $0, it'll reduce your corporate taxes by $3k, and means you can't rent the car or sell it. So why write it off, rather than sell it for $10k? $10k > $3k.