Your one machine's disk fails and you'll lose data if you have anything other than synchronous replication. And once you have synchronous replication let's hope it is on at least a different rack if in the same data-center. Preferably in a different geographical region actually. And now you no longer have "really fast" QPS.

All this assumes you have only so much data that can fit in one machine. If that's not the case you're in need of a database that spans more than one machine.