Costs

AWS have a vested (commercial) interest in making it as easy as possible for you to provision infrastructure. The benefit to you as the customer is that you can create and destroy these resources on demand.

Obviously, destroying infrastructure on-demand in a traditional, physical hardware environment is simply not possible. You would be hard-pressed to find a data center that will allow you to stop paying for servers and space simply because you are not currently using them.

Another use case where on-demand infrastructure can make large cost savings is your development environment. It only makes sense to have a development environment while you have developers to use it. When your developers go home at the end of the day, you can switch off your development environments so that you no longer pay for them. Before your developers come in in the morning, simply schedule their environments to be created.