Equalization of tax burden and transparency in reporting. To put it very simply: when you buy a thing you spend money and gain value. No change in books. Then after some time you sell said thing for generally less and experience sharp loss. Think of P&L report of a public company. Such maneuver will artificially increase profits during ownership and deflate, potentially up to incurring book loss, them in period of sale. Depreciation/amortization simply spreads this loss of value over the period of ownership. Tax implications are mostly incidental and come from reporting.