I got interested in AWS a while ago. Since its humble beginning as IaaS space, AWS has grown to be a the standard bearer for how to execute a worldwide IaaS and PaaS services.
I can think of many reasons you want to use AWS.
1) Best platform for standing up a test environment rapidly. Esp., for concurrency testing, load testing, multi-platform testing.
2) Quickly launch your consultant at client sites. Create AMIs with your desired platform configuration. Send your consultant to client site on a business class flight and Cadillac rental. With in 10 minutes on the site, your consultant can spin off a dev environment or a dev cluster through AWS.
3) Host legacy applications and DB.
4) Record management
6) Rent large scale computing power without large up front cost
5) The list goes on.
Great job, Amazon.
So far, I am impressed with that they have to offer