Clouds are very popular these days, because they provide a convenient virtual environment for a number of tasks from web hosting to scientific computing. In a cloud you can get a fresh ready to use system with almost any predefined parameters in a matter of minutes without need to purchase hardware and setup the OS. Besides that, clouds usually provide very fast and wide Internet connection. That is why it is usually a very good idea to use clouds for load testing of a web site. This is especially useful if the target site is hosted in the same cloud.
Since WAPT does not require any unusual hardware, it can be easily setup on any virtual system. There is no special license for this type of use.