The scaling policies detail how many instances to create or delete when they are triggered. It also defines a Cooldown value, which helps prevent flapping servers – when servers are created and deleted before they have finished starting and are useful. Often, an EC2 instance will have startup scripts that install third-party packages and custom application software that takes several minutes to complete. The scaling policies allow you to fine-tune the timing to avoid putting a machine into rotation before it is ready.
While the scaling policies in this example use equal values, you might want to change that so that your application can scale up quickly and scale down slowly for the best user experience.