The ability to scale your infrastructure through a combination of on- and off-site hosts is incredibly powerful. If you need more capacity, you can easily bring up new hosts on EC2, Rackspace Cloud, or any similar platform. Additionally, these techniques apply to not only public cloud providers but also to private cloud platforms such as VMWare vSphere and others (provided, a suitable plugin for knife exists).
As you have seen, with Chef you can spin up and spin down the server capacity to meet your needs with very little effort. Once your infrastructure management is automated, you can focus on higher level problems such as building scalable services and scaling to meet your customers' demands.
Expanding on this capability, you could use these tools to perform the following:
Manually increase or decrease the capacity in order to match the demand
Write a tool to analyze the current resource load and react accordingly
Predict the future capacity and scale appropriately on a given schedule...