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Chains are subject to extreme force, friction and abrasion. So are the teeth on the sprockets. As a result, they wear in sympathy with one another. Up to a certain point, this wear is within the tolerance of the system to cope with. If you change a chain frequently enough, or rotate three or four chains very regularly, you can fit a new chain and not have problems. In the majority of cases, however, changing the chain that frequently is not practical. The sympathetic wear pattern between sprockets and chain will exceed the new chain's ability to compensate: the tooth profile of the sprockets will have worn to the point where the new chain can no longer sit down fully into the teeth, and so will 'jump'. The only solution is then to change the sprockets for new ones. The same phenomenon works in reverse. The chain, as a result of friction and abrasion will 'stretch' making it unable to mesh correctly with the unworn teeth of new sprockets. Requiring that both are changed at the same time.
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