When Northwind buys a machine, cash leaves once and appears as capex in the cash-flow-statement. The income statement never sees that outflow; instead it sees a slice of the cost each year as depreciation, which also reduces the asset's carrying value in property-plant-and-equipment.
Because it is non-cash, depreciation is added back in operating-cash-flow. Useful-life assumptions are a judgement call, and lengthening them quietly raises reported profit.
Example: a $40M press with a ten-year life and no salvage value depreciates $4M a year. Stretching the assumed life to sixteen years cuts the annual charge to $2.5M and raises pre-tax profit by $1.5M with nothing else changing.
Related: amortisation, ebitda