Depreciation is designed to spread an asset’s cost over its entire useful service life. Its service life is the period over which it is worn out for any reason, at the end of which it is no longer usable, or not usable without extensive overhaul. Its... 

Fixed assets are those assets that are used in a productive capacity, are tangible (have physical substance), are relatively long-lived, and provide future benefit that is readily measurable, sometimes referred to as property, plant, and equipment [PP&E].... 

Inadequate provision for the maintenance of property, plant, and equipment detracts from the long-term earning power of the firm. If obsolete assets are not replaced and repairs not properly made, breakdowns and detracted operational efficiency will result....