Benefits of using cloud based systems (such as Amazon EC2 infrastructure)
Agility. Get up and running quickly, without requirements for servers and infrastructure. Yet, you still have full control over every aspect of the system.
Lack of up-front investment. Pay only for resources that are used.
Temporary situations or supplement to existing systems. Good environment for development, staging, testing, seasonal peak periods or new projects. Can also be used for backups of data, programs or entire servers. Systems can always be moved in-house for mission critical applications, or more permanent arrangements.