> I'm still not sure why Mt. Gox existed in the first place. Isn't the whole point of bitcoin that it is decentralized?

Insofar as the second sentence is true at all, it is in that the issuance and transaction verification of bitcoin and bitcoin transactions is decentralized.

That has nothing to do with the role of a centralized exchange in facilitating markets for people exchanging bitcoin for other commodities, like USD.