Who will buy - Virgin Australia ?, page-83

While the impairment of an assets carrying value may have a tax benefit, that benefit doesn’t usually get recognised in the tax return until the assets is sold or scrapped or the tax book value gets written down over time due to ongoing depreciation.

Where these timing differences occur, they give rise to a deferred tax asset (I.e. an asset that will be converted into cash in the future). To recognise (or to continue to recognise) a deferred tax asset, there needs to be a basis for arguing that tax will be payable in the future. If no tax is payable, the future deduction isn’t of value.