In a private network world you would have to build and pay for a lot of stuff yourself: multiple data centers, double the hardware, internet access connections on opposite sides of the building, etc. Very quickly the cost of high availability gets prohibitive, locking out all but the deepest of pockets.
With Cloud Computing, having access to redundant data centers is just a matter of purchasing the right performance monitoring tools and the engineering time in programming your applications and operational systems to take full advantage of on demand resources. In the end you only pay for what you use of the infrastructure, not what you might need as is the case when doing it yourself.
Cloud Computing is not outsourcing, this implies a transfer of risk and responsibility. Cloud Computing is a powerful tool to increase performance and availability many fold while reducing costs, if it’s used correctly.