There are many reasons why users choose Open Source Automation over other solutions. A big factor is that all the other major automation products available are very expensive. Usually running into hundreds, even thousands of dollars. OSA is free. Not only is it free, but it is open source as well so you can add your own features if desired. Another big reason to choose OSA is the extensibility. OSA is a built on a plugin framework that makes it very easy for anyone to develop plugins for any new service or hardware without requiring a lot of coding.
Open Source Automation supports many of the most popular automation hardware and many other less popular ones. There is a complete list here.
Open Source Automation runs on the Windows operating system. It does not require a very powerful machine and can even run on a virtual machine.
Support can be found in the forum.