One reason is 'currency' exchange. If you're mining crapcoins and want to exchange them into bitcoins, you need to do that through an exchange. It then often becomes easier just to leave the bitcoin on those exchanges. Also, if you're lazy and don't want to back up your wallet yourself, then theoretically having your currency on the exchange means it is accessible from anywhere and backed up.
Reality, of course, diverges from expectation.
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