There are cases (though rare) where exchanged delist or temporarily remove access to an asset. In this case, the API of the exchange may no longer provide any market data about said asset.
The specific use case that prompted this was BSV's temporary unavailability on Bittrex due to a recent 51% attack on the BSV chain. As a result a Kucoin was implemented as a fallback for Bittrex. Currently Bittrex is the only ccxt connection that has a default fallback, others may be implemented in the future.