> Satoshi Nakamoto stated in his white paper that: "The root problem with conventional currencies is all the trust that's required to make it work. The central bank must be trusted not to debase the currency, but the history of fiat currencies is full of breaches of that trust."

https://en.wikipedia.org/wiki/Bitcoin

It's clear that the purpose of Bitcoin is to replace existing banking institutions by providing a trust-less alternative. This means that using an exchange to store Bitcoin is essentially useless. If your purpose is to protect your money by handing it over to a trusted institution, then you're better off putting it into a bank that's FDIC insured.

Of course, the real reason that people store their Bitcoin on Coinbase is so that they can easily profit from speculation by exchanging their coin for USD.