A company buys a delivery truck for $50,000, paid in a single transaction. If that entire $50,000 hit the year's expenses all at once, that one year's profit would take a large, misleading hit, while every subsequent year the truck is still out there delivering packages would show no cost for it at all — an accounting picture that badly distorts how profitable the business actually was in any given year. Depreciation exists specifically to fix that distortion, spreading the truck's cost across every year it's actually expected to be useful, rather than dumping it all on the year of purchase.
The matching principle, applied to physical assets
Depreciation is a direct application of accounting's matching principle, which holds that expenses should be recorded in the same period as the revenue they helped generate. A truck expected to remain useful for ten years is, presumably, helping generate revenue across all ten of those years, not exclusively in the year it was purchased — so accounting spreads its cost across that same ten-year span, recording a portion of the original cost as a depreciation expense each year, rather than recognising the full cost upfront. This doesn't involve any actual additional cash leaving the business each year; the cash left once, at purchase. Depreciation is purely an accounting allocation, reflecting the asset's use up over time on the books, distinct from the cash flow itself.
Straight-line versus declining balance: two different assumptions about wear
The simplest depreciation method, straight-line depreciation, spreads an asset's cost evenly across its useful life — the same dollar amount expensed every year. Declining balance methods instead front-load more of the expense into the earlier years, reflecting an assumption that many assets, particularly vehicles and equipment, lose value and usefulness faster early in their life than later — a new truck loses more of its resale value in its first year than in its eighth, and some companies' depreciation schedules deliberately mirror that faster early decline rather than pretending the wear is perfectly even across every year. Amortisation applies the same underlying logic to intangible assets, like patents or licences, spreading their cost across their useful life the same way depreciation does for physical, tangible ones.
What we're still unsure about
The matching-principle logic behind depreciation, and the mechanics of straight-line and declining-balance methods, are standard, well-established accounting practice, not in dispute. What genuinely requires judgement, rather than a fixed formula, is estimating an asset's useful life and residual value in the first place — those estimates directly determine how much expense gets allocated to each year, and reasonable accountants can disagree about how long a given piece of equipment will actually remain useful, which is exactly why depreciation schedules, while systematic, still rest on assumptions that can turn out to be wrong in hindsight.
This sits inside Depreciation & Amortisation, one of seven topics in Accounting, one of four domains in Business, one of seventeen subjects the app can quiz you on.