Why is opportunity cost included in the computation? Also, why do we need to account for the tax effect of it?

Years 0 1 2 3 + ve + ve - ve -ve -ve + ve - ve -ve -ve o Sales (SP/unit x # units sold) Less Variable costs Less Relevant Fixed costs Less Depreciation (tax method) Less Loss on sale of asset -ve Add Gain on sale of asset +ve Less any incremental costs Less Annual opportunity costs -ve -ve -ve -ve -ve +ve -ve -ve -ve -ve +ve Net Profit before tax Tax at 20% Net Profit after taxes - ve/+ve +ve/-ve $ -ve $ +ve -ve $ -ve +ve $