Useful life is a legal classification for assets, usually in a business, which show the amount of time in years that an asset will be able to be used and the rate at which the asset's usefulness will depreciate.
The inclusion of useful life in a business accounting system allows the business to deduct the amount that the asset has depreciated each year from the business tax calculation. When the asset reaches the final year of depreciation, it can be "written off" or no longer considered as an asset.