It is a form of depreciation which is assumed to occur even when an item is used competently and with care and proper maintenance. For example, repeated impacts may cause stress to a hammer's head. In the normal use of a hammer for its designed task, this stress is impossible to prevent, and any attempt to eliminate it would make the hammer useless. At the same time, it is expected that the normal use of a hammer will not break it beyond repair until it has gone through a certain amount of use.
A company which supports a warranty on a product with the possibility of wear and tear will usually limit the warranty to a period of time where its wear and tear will probably not be enough to impede the use of the product significantly. Other factors such as the willingness of a customer to replace a product through warranty will affect how long the company offers it.